MB-11405 Reduce CPU use in active system

Instead of having all the threads snooze for the minimum
snooze time across all task sets, have only 1 thread per
task set snooze while others sleep for longer duration.
This not only uses lower cpu in an active system with 5+ buckets
but also reduces lock contention in smaller configurations.

Change-Id: I99a203c3d0fb36612fb884c44684f747d2f09e42
Reviewed-on: http://review.couchbase.org/38620
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>