ceph: use ceph_pagelist for mds reconnect message; change encoding (protocol change)

Use the ceph_pagelist to encode the MDS reconnect message.  We change the
message encoding (protocol change!) at the same time to make our life
easier (we don't know how many snaprealms we have when we start encoding).

An empty message implies the session is closed/does not exist.

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