Merge pull request #1610 from ceph/wip-4354-shared_ptr

Use shared pointers for Mutations/OpRequests in the MDS

Reviewed-by: Sage Weil <sage@inktank.com>
mds: fix shared_ptr MDRequest bugs

The main change is use shared_ptr instead of weak_ptr to define
active request map. The reason is that slave request needs to be
preserved until master explicitly finish it.

Fixes: #8026
Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>