rados: prevent ShardedOpWQ suicide_grace drop when waiting for work.

rados: prevent ShardedOpWQ suicide_grace drop when waiting for work.
The Sharded OpWQ will opportunistically wait for more work when
processing an empty queue. While waiting, the default work queue
heartbeat timeout and suicide_grace values are modified. The
`threadpool_default_timeout` grace is applied and suicide_grace is
disabled. If this op hangs, the heartbeat watchdog will not trigger an
OSD suicide recovery.

The default work queue values for grace and suicide_grace are re-applied
after finding work. This keeps the heartbeat timeouts consistent with
the values applied on _process() entry.

Fixes: https://tracker.ceph.com/issues/45076
Signed-off-by: Dan Hill <daniel.hill@canonical.com>