MOSDOpReply: add enough fields to be backwards compatible.

MOSDOpReply: add enough fields to be backwards compatible.

The system we've been building up works out very nicely for new clients,
but they could not have interoperated with old clients that were only
referring to our replay_version. In order to deal with this, we add
a bad_replay_version to MOSDOpReply which is encoded where we used
to encode replay_version. bad_replay_version will follow the same semantics
as reassert_version used to (except that it is filled in on reads), but
is not accessible to new clients, who can see only our properly-controlled
replay_version and user_version. This will let old and new clients
interoperate correctly when communicating about watches, etc.

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