ceph-common: Implementation of the sharded threadpool.

ceph-common: Implementation of the sharded threadpool.
Threadpool will only be having a single work queue and internally the
work queue will be having multiple storage data structures.
Based on some logic (which is derived class implementation specific)
the work queue will shard the requests among these storage structures.
Each storage will be guarded by finer grained sunchronization
objects. Sharded threadpool threads will be assigned to work on a
shard based on some algorithm which is again derived class implementation
specific.

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