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memtx: fix use-after-free on background index build

When building an index in background, we create on_rollback triggers for
tuples inserted concurrently. The problem here is on_rollback trigger
has independent from `index` and `memtx_ddl_state` lifetime - it can be
called after the index was build (and `memtx_ddl_state` is destroyed)
and even after the index was altered. So, in order to avoid
use-after-free in on_rollback trigger, let's drop all on_rollback
triggers when the DDL is over. It's OK because all owners of triggers
are already prepared, hence, in WAL or replication queue (since we
build indexes in background only without MVCC so the transactions cannot
yield), so if they are rolled back, the same will happen to the DDL.

In order to delete on_rollback triggers, we should collect them into a
list in `memtx_ddl_state`. On the other hand, when the DML statement is
over (committed or rolled back), we should delete its trigger from the
list to prevent use-after-free. That's why the commit adds the on_commit
trigger to background build process.

Closes #10620

NO_DOC=bugfix

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