mds: Delay session close if in clientreplay

If the mds is in clientreplay, a session close
request needs to be delayed until it reaches
active.  Otherwise, the session state gets set to
'closing', and the replay requests get dropped on the
floor.

Fixes #4564.
Signed-off-by: Sam Lang <sam.lang@inktank.com>
Merge pull request #159 from ceph/wip-4564

mds: Delay session close if in clientreplay

Reviewed-by: Sage Weil <sage@inktank.com>