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 finishes it.

Fixes: #8026
Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Merge pull request #1623 from ceph/wip-8026

mds: fix shared_ptr MDRequest bugs

Reviewed-by: Greg Farnum <greg@inktank.com>