mgr,mgr/MgrClient: use fsid to signal mon-mgr vs cli MCommands

mgr,mgr/MgrClient: use fsid to signal mon-mgr vs cli MCommands

We can't use the feature bit for the MCommand connection to tell whether
it is a tell or CLI command because new clients may have to send CLI
commands via MCommand for old clusters, and they don't always know whether
this mgr is new or old yet.

Prior to octopus, MCommand contained a mon/mgr CLI command, and did not
have the fsid field set.  Start populating the fsid field, and use this
to signal whether a client is a new MgrClient that knows MCommand vs
MMgrCommand.  If we get an MCommand with the fsid set, that means it is
a tell command; otherwise, it's an old client sending a CLI command.

Signed-off-by: Sage Weil <sage@redhat.com>