Reorganize ds_pool_child start/stop code to make it able to be
started/stopped individually:

- Introduced four states for ds_pool_child: NEW, STARTING, STARTED
  and STOPPING.
- ds_pool_child is added/removed to/from cache through the collective
  call of pool_child_add/delete_one(), now the cache doesn't hold the
  ds_pool_child reference anymore.
- Introduced ds_pool_child_start/stop() for callers to start/stop
  ds_pool_child individually, ds_pool_child_state() to query the state
  of ds_pool_child.
- Adjusted ds_pool_child_lookup() & ds_pool_child_get() a bit;

Required-githooks: true

Signed-off-by: Niu Yawei <yawei.niu@intel.com>

Address review comments from LiWei. Fix spelling errors.

Required-githooks: true

Signed-off-by: Niu Yawei <yawei.niu@intel.com>