Support 'expected_num_objects' parameter when creating pool for pg folder splitting

This patch adds a new parameter for pool creation named 'expected_num_objects', to give user a choice to specify the number of objects he/she would like to put into the pool. This information is used to pre-hash PG's namespace by creating a certain number of folders, so as to avoid run-time folder splitting which could saturate disk I/O.

Testing - Test against various combinations of PG number and object number. Use rados bench to put/get objects into the pool pre-hashed.

Signed-off-by: Guang Yang (yguang@yahoo-inc.com)