raft: fix race between snapshot application and committing of new entries
raft: fix race between snapshot application and committing of new entries

Completion notification code assumes that previous snapshot is applied
before new entries are committed, otherwise it asserts that some
notifications were missing. But currently commit notifications and
snapshot application run in different fibers, so the can be race between
those.

Fix that by moving commit notification into applier fiber as well.

Fixes #9550

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