mon: track intersection of quorum member features

mon: track intersection of quorum member features

When we form a quorum, also note the intersection of the quorum members'
feature bits.  This will inform decisions about what encodings we use.

This is an imperfect strategy because the quorum may change, and we may
have a mon with old code join in and not understand what is going on.
However, it does ensure that a majority of the members run new code, so in
the absence of other failures we can make progress.

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