mon/OSDMonitor: validate set-require-min-compat-client against connected clients

mon/OSDMonitor: validate set-require-min-compat-client against connected clients

Prevent the user from setting a require_min_compat_client if a disallowed
client is currently connected to the mon cluster.  Instead, you'll get an
error like

Error EPERM: cannot set require_min_compat_client to luminous: 1 connected client(s) look like jewel (missing 0x800000000200000); add --yes-i-really-mean-it to do it anyway

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