box: exclude uncommitted alter records from snapshot

With MVCC off (box.cfg.memtx_use_mvcc_engine = false), a memtx space
read view may include a dirty (not committed to WAL) record. To prevent
such records from being written to a snapshot, we sync WAL after
creating a read view for a snapshot. The problem is that it doesn't work
for long (yielding) DDL operations, such as building a new index,
because such operations yield before waiting on WAL. As a result,
a dirty DDL record may make it to a snapshot even though it may fail
eventually. To fix that, let's keep track of all yielding DDL statements
and exclude them from a read view using the memtx snapshot cleaner.

Closes #8530

NO_DOC=bug fix