Slow runtimes for ep-engine_ep_unit_tests under TSan

When running ep-engine_ep_unit_tests under ThreadSanitizer, the total test runtime is more than 10 minutes - for example:

00:19:20.852         Start  22: ep-engine_ep_unit_tests

00:30:53.164  31/177 Test  #22: ep-engine_ep_unit_tests ......................................................   Passed  692.31 sec

Job: http://cv.jenkins.couchbase.com/job/kv_engine-threadsanitizer-master/10499/console

Compare this to a normal Linux build which takes less than 30 seconds - TSan is ~20x slower:

00:08:35.129         Start  25: ep-engine_ep_unit_tests

00:08:58.527  27/198 Test  #25: ep-engine_ep_unit_tests ......................................................   Passed   23.38 sec

This isn't expected - TSan should generally be ~10x slower.

This is one of (if not the) slowest test when running under TSan - if we can improve it this should reduce the overall runtime of the TSan build.