ceph: wait for mds OPEN reply to indicate reconnect success

We used to infer reconnect success by watching the MDS state, essentially
assuming that hearing nothing meant things were ok.  That wasn't
particularly reliable.  Instead, the MDS replies with an explicit OPEN
message to indicate success.

Strictly speaking, this is a protocol change, but it is a backwards
compatible one that does not break new clients + old servers or old
clients + new servers.  At least not yet.

Drop unused @all argument from kick_requests while we're at it.

Signed-off-by: Sage Weil <sage@newdream.net>