task: mon_thrash: Thrash multiple monitors and 'maintain-quorum' option

task: mon_thrash: Thrash multiple monitors and 'maintain-quorum' option

We now add a new option 'thrash-many' that by being set to true will break
the default behaviour of killing only one monitor at a time.  Instead,
this option will select up to the maximum number of killable monitors to
kill in each round.

We also add a new 'maintain-quorum' option that will limit the amount of
monitors that can be killed in each thrashing round.  If set to true, this
option will limit the amount of killable monitors up to (n/2-1).  This
means that if we are running a configuration that only has up to two
configured monitors, if 'maintain-quorum' is set to true, this task won't
run as there are no killable monitors -- in such a scenario, this option
should be set to false.

Furthermore, if 'store-thrash' is set to true, then 'maintain-quorum' must
also be set to true, as we cannot let the task to thrash all the monitor
stores, or we wouldn't be able to sync from other monitors, nor can we
let quorum be dropped, or we won't be able to resync our way into quorum.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>