mon: fix issues with mixed-version monitors features

Mainly, we were setting features in a manner that would have a freshly
mkfs'ed monitor to write all the features it supports to disk.  This would
make it impossible to form a quorum with an older monitor, even though
it should -- say, quorum does not support feature X but monitor, having
been mkfs'ed with feature X, would expect given feature to be available
in the quorum.

Also fixes a few other issues, including returning the proper features in
Monitor::get_required_features() -- we were returning 'quorum_features'
instead.