memtx: add yields during space format check

Just like index build, space check in memtx stalls event loop for the
whole check time. Add occasional yields, and an on_replace trigger,
which checks format for tuples inserted while space format check is in
progress.

Follow-up #3976

@TarantoolBot document
Title: memtx now checks space format in background

There is no event loop stall when memtx engine checks space format
anymore. You may insert tuples into a space, while its new format is
being checked. If the tuples don't match the new format, format change
will be aborted.