memtx: invalidate index right on DDL in MVCC

Currently, we delete MVCC trackers from index when it's being deleted.
It doesn't actually break anything but we can do it right on DDL since
all concurrent transactions are aborted and trackers are not needed
anymore. Also, having a saved key belonging to old key def (it's saved
by gap inplace item) is not a good idea and can lead to sudden bugs
in future - let's just delete all the trackers right on DDL while we
are here.

Follow-up #10146

NO_TEST=refactoring
NO_CHANGELOG=refactoring
NO_DOC=refactoring

(cherry picked from commit 8879489834f39fdb46093c1988fbf4f1601d51ff)