Synchronize naming with the latest C++ standard.

Needed for #1848

This patch doesn't introduce any functional changes.

See #1585

Closes #1585

Closes #1131

* added 'ignore_crc' flag to xlog_cursor
* added tests for bad crc, bad version, bad format, good v12/v13 fformat reading

+ Review fixes from rtsisyk

Fixes #1854

Currently, an error is propagated from Engine::{begin,wait}Checkpoint()
to lbox_snapshot() in errno. This is ugly and inflexible, as we are only
allowed to throw SystemError from Engine - see how
MemtxEngine::waitCheckpoint converts diag to SystemError in order to
extract errno. We should use diag instead of errno all the way from
Engine to lbox.

For reporting about checkpoint-in-progress, use ClientError with a new
code, ER_SNAPSHOT_IN_PROGRESS, instead of EINPROGRESS errno, as it's
obviously not a SystemError.

Closes #1858

See #1572

xlog for each recovey after new xlog file and pread implementation.
Optimize fake eof (not closed xlog) cause for recovery.

xlog for each recovey after new xlog file and pread implementation.
Optimize fake eof (not closed xlog) cause for recovery.

None of existing task kinds return anything but 0 on completion.
So let's remove the unused ret code to simplify error handling.

Use SystemError and ClientError where appropriate. Note, ER_VINYL is
still there - it is used for reporting about corrupted/incomplete range
files on recovery. However, as soon as we switch to the xlog file
format, we'll probably be able to get rid of it altogether.

We need it to propagate system errors from vinyl.

Closes #1713

Test may fail on OS X due to NOFILE limit.

vy_read_iterator->curr_stmt is supposed to store the last statement
returned by vy_read_iterator_next() to be used for iterator restore.
However, currently it is also updated in vy_read_iterator_next_range().
If the iterator is restored after such an update it will skip the first
statement in the range, which might result in invalid select result. For
instance, if the first statement (the newest one) in a range is a delete
followed by an upsert for the same key (which is older), we will skip
the delete on restore, and return the deleted key, see

  vy_read_iterator_restore:
    vy_merge_iterator_restore:
      vy_run_iterator_restore:

        if (vy_stmt_compare(fnd, last_stmt->data, itr->index->key_def) == 0) {
                position_changed = false;
                if (next_stmt->lsn >= last_stmt->lsn) {
                        /* skip the same stmt to next stmt or older version */

This results in occasional vinyl/options test failures.

Closes #1853

Conflicts:
	test/unit/bps_tree.cc
	test/unit/bps_tree_iterator.cc

Allow to specify allocator context in matras and matras-based data
structures. After this change, the following salad data structures can
be passed a custom allocator context: light, rtree, bps_tree.

Currently, we use a rather dumb way to determine if a statement from
xlog should be committed or not on recovery - we lookup the statement in
all on-disk runs and, if found, compare its lsn. Obviously, this slows
down the recovery procedure significantly. Actually, we can avoid
reading on-disk runs altogether. All we need to do is compare the xlog
statement's lsn with the range's max lsn - we should only commit the
statement if the former is greater.

Closes #1697

Whenever we need VINYL_EQ order in range iterator, we use VINYL_GE.
This behavior exists solely due to historical reasons and needs to
be fixed.

Currently, we never remove delete statements on dump, even if creating
the first run. However, there's no point in preserving deletes in case
there's no on-disk data.

Closes #1823

Fixes #1442

Surround key and lsn in parentheses.

If there are a lot of successive upserts for the same key, select might
take too long to squash them all. So once the number of upserts exceeds
a certain threshold, we schedule a fiber to merge them and substitute
the latest upsert with the resulting replace statement.

Closes #1829