mds: revise mds sessionmap encoding [disk format change]
mds: revise mds sessionmap encoding [disk format change]

Encode session name before session itself, so that we can use
an existing session instead of allocating a new one.  This lets
us keep eagerly reconnecting clients that connect before we
load the sessionmap.

Add proper struct_v.  Drop useless/incorrect 'n' value.

Continue to read old format, of course.  Some minor hackery
because we didn't have a struct_v before.

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph