client: handle reset during initial mds session open

If we get a reset during our attempt to open an MDS session, close out the
Connection* and retry to open the session, moving the waiters over.

Fixes: #5379
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>
(cherry picked from commit df8a3e5591948dfd94de2e06640cfe54d2de4322)