rados: expand mon thrashing tests

- test sync (full and recent)
- test multiple mons down
- test other cluster sizes (3 and 9)

Signed-off-by: Sage Weil <sage@inktank.com>
rados/monthrash: refactor a bit

Signed-off-by: Sage Weil <sage@inktank.com>
rados/monthrash: reduce matrix size a bit

we don't need so many osd workloads
rados/mon_thrash: inject random delays in mon messages

More useful than osd delays for this collection