MB-19246: Fix potentially incorrect persist_time in OBSERVE response

MB-19246: Fix potentially incorrect persist_time in OBSERVE response

There is a data race in updating EPStore::lastTransTimePerItem. This
could result in an incorrect value for the persist_time field in an
OBSERVE response.

WARNING: ThreadSanitizer: data race (pid=4590)

  Write of size 8 at 0x7d540000fe88 by thread T8 (mutexes: write M11599):
    #0 EventuallyPersistentStore::flushVBucket(unsigned short) /home/abhinav/couchbase/ep-engine/src/ep.cc:3307 (ep.so+0x00000009954f)
    #1 Flusher::flushVB() /home/abhinav/couchbase/ep-engine/src/flusher.cc:296 (ep.so+0x0000000ff32f)
    #2 Flusher::step(GlobalTask*) /home/abhinav/couchbase/ep-engine/src/flusher.cc:186 (ep.so+0x0000000fd825)
    #3 FlusherTask::run() /home/abhinav/couchbase/ep-engine/src/tasks.cc:63 (ep.so+0x00000013bbb2)
    #4 ExecutorThread::run() /home/abhinav/couchbase/ep-engine/src/executorthread.cc:112 (ep.so+0x0000000f89b6)
    #5 launch_executor_thread(void*) /home/abhinav/couchbase/ep-engine/src/executorthread.cc:33 (ep.so+0x0000000f8555)
    #6 platform_thread_wrap /home/abhinav/couchbase/platform/src/cb_pthreads.c:23 (libplatform.so.0.1.0+0x000000003d31)

  Previous write of size 8 at 0x7d540000fe88 by thread T6 (mutexes: write M11602):
    #0 EventuallyPersistentStore::flushVBucket(unsigned short) /home/abhinav/couchbase/ep-engine/src/ep.cc:3307 (ep.so+0x00000009954f)
    #1 Flusher::flushVB() /home/abhinav/couchbase/ep-engine/src/flusher.cc:296 (ep.so+0x0000000ff32f)
    #2 Flusher::step(GlobalTask*) /home/abhinav/couchbase/ep-engine/src/flusher.cc:186 (ep.so+0x0000000fd825)
    #3 FlusherTask::run() /home/abhinav/couchbase/ep-engine/src/tasks.cc:63 (ep.so+0x00000013bbb2)
    #4 ExecutorThread::run() /home/abhinav/couchbase/ep-engine/src/executorthread.cc:112 (ep.so+0x0000000f89b6)
    #5 launch_executor_thread(void*) /home/abhinav/couchbase/ep-engine/src/executorthread.cc:33 (ep.so+0x0000000f8555)
    #6 platform_thread_wrap /home/abhinav/couchbase/platform/src/cb_pthreads.c:23 (libplatform.so.0.1.0+0x000000003d31)

Change-Id: Iccf1b0eacba495a8147fe81922361d566cb1d6a0
Reviewed-on: http://review.couchbase.org/62967
Well-Formed: buildbot <build@couchbase.com>
Reviewed-by: Will Gardner <will.gardner@couchbase.com>
Tested-by: buildbot <build@couchbase.com>