mon/OSDMonitor: fix osdmap encode feature logic

If we are encoding a full map based on an old Incremental that does not encode
the features, fall back to the quorum features or (barring that) all features.
 Do *not* do no features or else we will end up with encode_client_old which
does not even include the extended info and will cause the mon to crash when
decoding.

This was observed when upgading a 0.76 cluster to 0.77 (all mons stopped, 
upgraded, and then started)

Reported-by: Aaron Ten Clay <aarontc@aarontc.com> Signed-off-by: Sage Weil
<sage@inktank.com>