Message,OSD,PG: make Connection::features private

Use has_feature() method too.

Signed-off-by: Samuel Just <sam.just@inktank.com>
Signed-off-by: Sage Weil <sage@inktank.com>
msg/Pipe: work around incorrect features reported by earlier versions

If we see a peer reporting features ~0ull, we know they are deluded in a
particular way and should infer what features they *actually* have.  Do
this right when the features come over the wire to catch all users.

Fixes: #5655
Signed-off-by: Samuel Just <sam.just@inktank.com>
Signed-off-by: Sage Weil <sage@inktank.com>