Monitor: Don't skip some statistics, speed up

_wait_for_empty_queues was modifying the {metrics} list when iterating
over it, which could cause some items to not be checked. This had the
effect of waiting more often than necessary when checking for zero
queues.

Change-Id: I9d7b62efb24dc43739e58800423fa56a11a3bc47
Reviewed-on: http://review.couchbase.org/39946
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>