mon_thrash: optionally scrub after each iteration (default true)

Backported from master, with small change that we ignore failure of the
scrub command, in case it isn't present.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 5f1f0cb9a7f27eb30c631a343b1f7624505d184e)