backward incompatible pg_basebackup and pg_receivexlog

pg_basebackup and pg_receivexlog from 9.3 won't work with earlier
servers anymore.  I wonder if this has been fully thought through.  We
have put in a lot of effort to make client programs compatible with many
server versions as well as keeping the client/server protocol compatible
across many versions.  Both of these assumptions are now being broken,
which will result in all kinds of annoyances.

It seems to me that these tools could probably be enhanced to understand
both old and new formats.

Also, using the old tools against new server versions either behaves
funny or silently appears to work, both of which might be a problem.

I think if we are documenting the replication protocol as part of the
frontend/backend protocol and are exposing client tools that use it,
changes need to be done with the same rigor as other protocol changes.
As far as I can tell, there is no separate version number for the
replication part of the protocol, so either there needs to be one or the
protocol as a whole needs to be updated.