mon: mkfs compatset may be different from runtime compatset

mon: mkfs compatset may be different from runtime compatset

When we create a monitor we set a given number of compat features on
disk to clearly state the features a given monitor supports -- mostly to
break backward compatibility when such compatibility cannot be
guaranteed.

However, we may wish to toggle some features during runtime; e.g., wait
for all the monitors in the quorum to support a given feature before
flipping a switch and state that all monitors now require feature X.

We are already flipping those switches during runtime, but we weren't
allowing the monitor to set a subset of those features during mkfs.
While the initial approach worked fine with clusters being upgraded and
fresh clusters, it could become weird in a mixed-version environment.

Backport: emperor,firefly,giant

Signed-off-by: Joao Eduardo Luis <joao@redhat.com>