MB-16500 [BP]: Address possible data race in checkpoint remover

MB-16500 [BP]: Address possible data race in checkpoint remover

WARNING: ThreadSanitizer: data race (pid=102986)

  Read of size 1 at 0x7d180000c298 by thread T17:
    #0 ClosedUnrefCheckpointRemoverTask::run() /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/checkpoint_remover.cc:139 (ep.so+0x00000003d4a6)
    #1 ExecutorThread::run() /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/executorthread.cc:115 (ep.so+0x0000000f9c4c)
    #2 launch_executor_thread(void*) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/executorthread.cc:33 (ep.so+0x0000000f9815)
    #3 platform_thread_wrap /home/couchbase/.ccache/tmp/cb_pthread.tmp.9515862b2292.83537.i:0 (libplatform.so.0.1.0+0x0000000041b1)

  Previous write of size 1 at 0x7d180000c298 by thread T18:
    #0 CheckpointVisitor::complete() /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/checkpoint_remover.cc:69 (ep.so+0x00000003e0a1)
    #1 VBCBAdaptor::run() /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/ep.cc:3789 (ep.so+0x00000009d64a)
    #2 ExecutorThread::run() /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/executorthread.cc:115 (ep.so+0x0000000f9c4c)
    #3 launch_executor_thread(void*) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/executorthread.cc:33 (ep.so+0x0000000f9815)
    #4 platform_thread_wrap /home/couchbase/.ccache/tmp/cb_pthread.tmp.9515862b2292.83537.i:0 (libplatform.so.0.1.0+0x0000000041b1)

Change-Id: I8579d5af259490e41028f57f302e547b0826fa61
Reviewed-on: http://review.couchbase.org/55937
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-on: http://review.couchbase.org/56067