repair: Wait for pending repair_meta operation before removing it
repair: Wait for pending repair_meta operation before removing it

We remove repair_meta object in remove_repair_meta up receiving of stop
row level repair rpc verb. It is possible there is an pending operation
of repair_meta. To avoid use after free, we should not remove the
repair_meta object until all the pending operations are done.
Use a gate to protect it.

Fixes: #4333
Fixes: #4314
Tests: 50 succesful run of repair_additional_test.py:RepairAdditionalTest.repair_kill_2_test

scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla