mon_thrash: optionally scrub after each iteration (default true)

Signed-off-by: Sage Weil <sage@inktank.com>
daemon-helper: send arbitrary signals via stdin

Each byte written to stdin will be interpreted as a signal.

Signed-off-by: Sage Weil <sage@inktank.com>
mon_thrasher: add pause/unpause of mons to thrashing

This adds an additional element of laggyness to the cluster which should
cause mons to call new elections.

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