mon: implement simple 'scrub' command

Compare all keys within the sync'ed prefixes across members of the quorum
and compare the key counts and CRC for inconsistencies.

Currently this is a one-shot inefficient hammer.  We'll want to make this
work in chunks before it is usable in production environments.

Protect with a feature bit to avoid sending MMonScrub to mons who can't
decode it.

Signed-off-by: Sage Weil <sage@inktank.com>