Description of problem:

Previously, snap trimming and scrub happened in a background thread pool meaning that scrub and snap trimming were scheduled vs client ops at the whim of the os scheduler.  Instead, move them into the same queue with lower priority so that the queue implementation can prioritize client IO directly while avoid starvation.

The main things to test are:
1) scrub throughput is adequate when cluster is not loaded

For this, populate a cluster to let's say 50% full of a mix of rbd and radosgw objects (make sure no single radosgw bucket gets unreasonably large).  Initiate a scrub, make sure the progress is reasonable (doesn't hang, comparable to hammer completion time).  Scrubbing a single pg on a 3 osd cluster is probably adequate.

2) client IO latency doesn't suffer too much (hopefully improves!) compared with hammer during a scrub (deep and shallow)

Populate the cluster as above.  Pick a few precreated rbd images and run fio small writes. Record 99th percentile latency and average throughput while scrubbing (you can probably just set the scrub interval to be very so that they are always scrubbing).  Verify that these do not suffer compared with hammer.