[MERGE][0.16] Allow new smart client to work with old (<= 0.15) smart servers.

[Andrew Bennetts]

After a suggestion from Robert, this bundle will allow the new hpss clients to
work with old smart servers again.  It does a version probe in
RemoteBzrDirFormat.probe_transport, and if the server isn't new enough then the
new hpss logic won't be used — which allows the old VFS-only "smart" operations
to be tried instead.

i.e. you can do "bzr log bzr+ssh://" with new bzr on the client, and bzr 0.15
(or even 0.11) on the server and it will Just Work (although no quicker than if
you had bzr 0.15).

-Andrew.