ceph-common: The Sharded threadpool worker logic changed
Now, the _process() of the derived queue is processing one request
at a time and the outer loop is controlled by the sharded threadpool.
The stop/pause/drain functionalities are controlled by the sharded TP.

Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>
OSD:Derived sharded queue implementation is changed
All the threadpool related stuff like stop/pause/drain etc. are not
handled by sharded queue anymore. All it is implementing are related to
processing,enqueue , signaling of waiting threads and shard queue status.
The pg ordering is been taken care of by introducing a map <pg,op> in each
shard.

Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>