* use per-index statistics
* remove step_count as it is no longer maintained
* add statistics for txw, mem, and index overall

Old iterator has several problems:

- restoration system is too complex and might cause several
  reads from disk of the same statements.

- applying of upserts works in direct way (squash all upserts
  and apply them to terminal statement) and the code doesn't
  leave a change to optimize it.

Implement iterator for full-key EQ case that fixes problems above.

There is a version member in vy_index that is incremented on each
modification of mem list and range tree. Split it to two members
that correspond to mem list and range tree accordingly.

It is needed for more precise tracking of changes in iterators.

If a vylog record doesn't get flushed to disk due to an error, objects
it refers to (index->key_def, range->begin and range->end) may get
destroyed, resulting in a crash. To avoid that, we must copy those
objects to vylog buffer.

Closes #2532

 - Remove tx and cursor latencies as they are useless - they actually
   account how long a tx/cursor was open, not latencies.
 - Remove vy_stat->get_latency as it doesn't account latency of select,
   besides we now have per index read latency.
 - Remove vy_stat->dumped_statements and dump_total as these statistics
   are reported per index as well.
 - VY_STAT_TX_OPS is currently unused (always 0). Let's use it for
   accounting the total number of statements committed in tx instead of
   vy_stat->write_count.

index.info() is supposed to show index stats, not options.
box.space.<space_name>.index.<index_name>.options looks like
a better place for reporting index options.

Needed for #1662

This patch adds 'latency' field to index.info. It shows the latency of
reads from the index. The latency is computed as 99-percentile of all
delays incurred by vy_read_iterator_next().

Needed for #1662

Replace box.info.vinyl().performance.upsert_{squashed,applied} with per
index index.info().upsert.{squashed,applied}.

Needed for #1662

Add the following counters to index.info:

  lookup                # number of lookups (read iter start)
  get                   # number of statements read (read iter next)
    rows
    bytes
  put                   # number of statements written
    rows
    bytes

Needed for #1662

closes gh-2516

This patch adds the following counters to index.info:

  txw
    count               # number of statements in the TX write set
      rows
      bytes
    iterator
      lookup            # number of lookups in the TX write set
      get               # number of statements returned by the iterator
        rows
        bytes

Needed for #1662

This patch adds the cache section to index.info with the following
counters in it:

  cache
    rows                # number of tuples in the cache
    bytes               # cache memory size
    lookup              # lookups in the cache
    get                 # reads from the cache
      rows
      bytes
    put                 # write to the cache
      rows
      bytes
    invalidate          # overwrites in the cache
      rows
      bytes
    evict               # evictions due to memory quota
      rows
      bytes

Needed for #1662

Using vy_quota, which was implemented to support watermarking,
throttling, timeouts, for accounting cached tuples is an overkill.
Replace it with mem_used and mem_quota counters.

This patch adds the following counters to the disk section index.info:

  dump                  # dump statistics:
    count               #   number of invocations
    in                  #   number of input statements
      rows
      bytes
    out                 #   number of output statements
      rows
      bytes

  compact               # compaction statistics:
    count               #   number of invocations
    in                  #   number of input statements
      rows
      bytes
    out                 #   number of output statements
      rows
      bytes

Needed for #1662

Replace box.info.vinyl().performance.iterator.{run,mem} global counters
with the following per index counters:

  memory
    iterator
      lookup            # number of lookups in the memory tree
      get               # number of statements returned by mem iterator
        rows
        bytes

  disk
    iterator
      lookup            # number of lookups in the page index
      get               # number of statements returned by run iterator
        rows
        bytes

      bloom             # number of times bloom filter
        hit             #   allowed to avoid a disk read
        miss            #   failed to prevent a disk read

      read              # number of statements actually read from disk
        rows
        bytes
        bytes_compressed
        pages

Needed for #1662

It's useless - it only checks that certain counters are present, but
doesn't give a damn about what they show. At the same time, I have to
update its output every time I modify vinyl statistics, which I'm doing
pretty often these days. So let's ditch it - I'll rewrite it from
scratch after I'm done reworking vinyl statistics.

gh-2520: add comments explaining the test case for gh-2520 (upsert
caching).

Use a tuple cache to turn an UPSERT in the PREPARE, if the cache
contains a tuple with the exact same key. The UPSERT can be applied
to the cached tuples, because
1. The cache always contains only REPLACE statements;
2. The cache always contains only newest statements.

Closes #2520

When an UPSERT is prepared, it can use a cached statement to be
turned into REPLACE. But now it doesn't use the cache.

Test for #2520

Minor style fixes to improve code readability. In scope of gh-2529.

Do not unlock endpoint until a flushing async message is sent and do not
destroy endpoint until last pipe release endpoint lock. Fixed #2529

* use name_len and sz consistently across all patches working with names
  and lengths
* do not delete user_def for a system user during init
* use calloc(), since we're using it for index_def as well.

Need for #944

In scope of gh-2547 (lost of space access control lists during truncate).

* use an existing index_def from the new space instead.

IMO makes the code a bit easier to follow.

In scope of work on gh-944 (variable length object names).

* we use name_len for name length in most of the places,
  len is a widespread abbreviation for length

* add comments

Needed for #944

Malformed MessagePack can cause `int k` counter overflow
inside mp_check()/mp_next().

See https://github.com/tarantool/nginx_upstream_module/issues/79
See https://github.com/rtsisyk/msgpuck/issues/16
Closes #2540