Always check return code from next_key()/next_lsn()/get().

Page always must have at least one tuple.

Follow-up for d1e4b531 ("vinyl: remove aging from scheduler").

Get rid of harmful heuristic.

To catch bugs like the one fixed by 05423bcc ("vinyl: fix small run
recovery").

If run size is less than ALIGN_POS(sizeof(struct vy_run_info)), then
vy_range_recover() won't load it. This is due to a residue of aligned
read/write functionality, the rest of which was removed by commit
7fe24c42 ("All operation will be done over xlog. We don't need to use
aligned rw ops"). Remove this last ALIGN_POS. Steps to reproduce:

  space = box.schema.space.create("vinyl", { engine = 'vinyl' })
  space:create_index('primary', { parts = { 1, 'unsigned' } })
  space:insert({0})
  box.snapshot
  -- restart server
  space:select()

We don't really need to init vy_range->min_key each time we insert it to
a tree, as we do now. It's enough to do it once in a range's lifetime -
when the range is created, i.e. on recovery or on split.

While we're at it, let's not panic if we fail to alloc a tuple for
min_key, but fail gracefully.

grep_log() now checks a bigger chunk of log by default (64K vs 2K), so
that debug tracing shouldn't normally break it. If it does, one can
increase grep_log's bytes argument manually to check even more.

Follow up 3dc06ff8 and
          b7724150

Also removed vlsn from vy_run_iterator_search

We shouldn't link *empty* mems back to the original range, but we don't
link *non-empty* ones due to the typo.

* Replace vy_run->page_cache with LRU cache in vy_run_iterator
* Remove vy_run_iterator_lock_page()/vy_run_iterator_unlock_page()
* Refactor struct vy_page to remove dangling pointers to vy_page_info
* Fix error handling in vy_run_iterator and vy_merge_iterator
* Remove special optimization for case when key == page->min_key

Currently, a dropped index might have compaction scheduled on scheduler
stop, in which case its ranges are inter-linked and cannot be easily
freed (dumb free of all ranges would result in double free). So let's
drop the final index cleanup for now. We need to rework it anyway,
because we never free active indexes on shutdown, although we should to
make valgrind happy.

Currently, compaction works as follows:
 1. worker: write old range's runs and shadow indexes to disk, creating
    as many new ranges as necessary (min 1)
 2. tx: redistribute active memory index of the old range among new
    ranges
 3. tx: replace old range with new ones and delete old one

Such a design has a serious drawback: redistribution (step #2) scales
lineary with the number of tuples and hence may take too long.

So this patch reworks the compact procedure. Now it looks like this:
 1. tx: create new ranges and insert them to the tree instead of the old
    range; in order not to break lookups, link mem and run of the old
    range to new ones
 2. worker: write mem and run of the old range to disk creating runs for
    each of new ranges
 3. tx: unlink old range's mem and run from new ranges and delete it

An old range is either split in two parts by the median key
(approximate) or not split at all, depending on its size.

Note, we don't split a range if it hasn't been compacted at least once.
This breaks assumptions of vinyl/split test, so I disable it for now. I
will rework it later, perhaps after sanitizing the scheduler.

Closes #1745

Reorder the branches of squash loop to begin with REPLACE,
this makes the reasoning about this loop a whole lot easier.

Always use vy_tuple_key_part(key, 0) == NULL
as an indicator of NULL key.

Before this patch, some of the edge case optimizations in
run, mem and txv iterators didn't work, since they compared
input key with NULL, whereas it was set to a special key with zero
parts.

@todo: this is a yet another discrepancy stemming from distinct
tuple formats in memtx and vinyl.

Index page size and range size are storage-level properties
and can not change during index life time.
Initialize these properties from global defaults at index
creation time. This way they are not affected after restart
with different global options.

Remove "engine-level" options support from Lua, all options
are known to us now that we ditched third party engines.

Closes #1738

Closes #1737

g/static inline/statis is easier than merging an old branch
with a runaway trunk