vinyl: fix compaction vs checkpoint race resulting in invalid gc

The callback invoked upon compaction completion uses checkpoint_last()
to determine whether compacted runs may be deleted: if the max LSN
stored in a compacted run (run->dump_lsn) is greater than the LSN of the
last checkpoint (gc_lsn) then the run doesn't belong to the last
checkpoint and hence is safe to delete, see commit 35db70fa ("vinyl:
remove runs not referenced by any checkpoint immediately").

The problem is checkpoint_last() isn't synced with vylog rotation - it
returns the signature of the last successfully created memtx snapshot
and is updated in memtx_engine_commit_checkpoint() after vylog is
rotated. If a compaction task completes after vylog is rotated but
before snap file is renamed, it will assume that compacted runs do not
belong to the last checkpoint, although they do (as they have been
appended to the rotated vylog), and delete them.

To eliminate this race, let's use vylog signature instead of snap
signature in vy_task_compact_complete().

Closes #3437