mon: note mds beacon times more carefully

mon: note mds beacon times more carefully

We need to update the beacon timestamp even when we are updating
the mds state.  Otherwise we can get caught in a busy loop
between marking an mds laggy and !laggy because the beacon stamp
never updates.

So even if we are updating, and the reply will be slow, update
our timestamp, so we don't mark the mds laggy.