mds: cap_reconnect_t uses modern encoding

Signed-off-by: Greg Farnum <greg@inktank.com>
mds: add ENCODING to the incompat set

Signed-off-by: Greg Farnum <greg@inktank.com>
mds: remove very dead commented-out code

Signed-off-by: Greg Farnum <greg@inktank.com>
mds: enable SnapInfo, snaplink_t, sr_t dencoder usage

Signed-off-by: Greg Farnum <greg@inktank.com>