magma: Flushing performance of ep_testsuite_basic 'multi set' ~5x slower than Couchstore

Seeing unit test failure for magma only, failures relate to ep_queue_size and occured in 'multi set' test for value and full eviction buckets

http://cv.jenkins.couchbase.com/job/kv_engine.threadsanitizer/job/master/4440/

ep_testsuite_basic.full_eviction.magma

16:41:04  Running [0007/0047]: multi set...Exceeded maximum wait time of 60000000us waiting for stat 'ep_queue_size' to be 0 (last value:73650) - aborting.

i.e. within the allotted 60s deadline, only ~25% of the 100,000 items have been flushed. No such timeouts have been seen with Couchstore.