mds: Add encode/decode/dump for use with dencoder

This is for the MDSTable subclasses: AnchorServer,
SnapServer, InoTable.  AnchorServer was partially there
but didn't work in practice dumped objects because of
the version prefix on the object.

This adds a skip argument to ceph-dencoder for getting
past fixed size prefixes.  The encode_server_state method
is also made const to avoid the need for const_casting (and
because it really is const)

Signed-off-by: John Spray <john.spray@inktank.com>