Intermittent test failure: "multi set...Exceeded maximum wait time of 60000000us waiting for stat 'ep_queue_size' to be 0" (14%)

Error signature

Failure of an engine_testapp test:

<pre>

Running [X/Y]: multi set...Exceeded maximum wait time of 60000000us waiting for stat 'ep_queue_size' to be 0 (last value:XXX) - aborting.

</pre>

Details
26 instances of this failure (14.3% of sampled failures):

• Time: 2020-11-27 12:02:47, Jenkins job: http://cv.jenkins.couchbase.com/job/kv_engine.threadsanitizer/job/master/7784/, patch: http://review.couchbase.org/c/kv_engine/+/140427
• Time: 2020-11-27 07:37:28, Jenkins job: http://cv.jenkins.couchbase.com/job/kv_engine.threadsanitizer/job/master/7768/, patch: http://review.couchbase.org/c/kv_engine/+/140036
• Time: 2020-11-26 18:15:34, Jenkins job: http://cv.jenkins.couchbase.com/job/kv_engine.threadsanitizer/job/master/7761/, patch: http://review.couchbase.org/c/kv_engine/+/141044
• Time: 2020-11-26 16:47:15, Jenkins job: http://cv.jenkins.couchbase.com/job/kv_engine.threadsanitizer/job/master/7759/, patch: http://review.couchbase.org/c/kv_engine/+/141021
• Time: 2020-11-26 16:47:10, Jenkins job: http://cv.jenkins.couchbase.com/job/kv_engine.threadsanitizer/job/master/7756/, patch: http://review.couchbase.org/c/kv_engine/+/141018
• Time: 2020-11-26 16:28:28, Jenkins job: http://cv.jenkins.couchbase.com/job/kv_engine.threadsanitizer/job/master/7754/, patch: http://review.couchbase.org/c/kv_engine/+/141058
• Time: 2020-11-26 16:28:28, Jenkins job: http://cv.jenkins.couchbase.com/job/kv_engine.threadsanitizer/job/master/7750/, patch: http://review.couchbase.org/c/kv_engine/+/140962
• Time: 2020-11-26 16:28:28, Jenkins job: http://cv.jenkins.couchbase.com/job/kv_engine.threadsanitizer/job/master/7748/, patch: http://review.couchbase.org/c/kv_engine/+/140956
• Time: 2020-11-26 15:26:52, Jenkins job: http://cv.jenkins.couchbase.com/job/kv_engine.threadsanitizer/job/master/7746/, patch: http://review.couchbase.org/c/kv_engine/+/141055
• Time: 2020-11-26 14:15:21, Jenkins job: http://cv.jenkins.couchbase.com/job/kv_engine.threadsanitizer/job/master/7742/, patch: http://review.couchbase.org/c/kv_engine/+/140036
  <cut> - only showing details of first 10/26 instances.