Add a new field 'expected_num_objects' to pg_pool_t which denotes the expected number of objects on this pool.

Signed-off-by: Guang Yang (yguang@yahoo-inc.com)
Add a new monitor command to let user specify the expected number of objects during pool creation.

Signed-off-by: Guang Yang (yguang@yahoo-inc.com)
Add a new transaction OP (collection hint) to ObjectStore.
Signed-off-by: Guang Yang (yguang@yahoo-inc.com)
Implement the collection hint transaction, add a new transation type as expected number of objects.
Signed-off-by: Guang Yang (yguang@yahoo-inc.com)
Add tests for the collection hint OP: 1) Store Test 2) Idempotent Test.
Signed-off-by: Guang Yang (yguang@yahoo-inc.com)
Merge pull request #2043 from guangyy/wip-pg-splitting

Support 'expected_num_objects' parameter when creating pool for pg folder splitting

Reviewed-by: Samuel Just <sam.just@inktank.com>