mon: decline to scrub when paxos is not active

In f1ce8d7c955a2443111bf7d9e16b4c563d445712 we close a race between scrub
and paxos commit completion on the leader.  The fix is nontrivial to
backport and probably not worthwhile; just avoid scrubbing at that time
for now.

Note that the actual fix for this is in commit
f1ce8d7c955a2443111bf7d9e16b4c563d445712.

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