MB-19252: Fix data race on Stream::readyQueueMemory

MB-19252: Fix data race on Stream::readyQueueMemory

As detected by TSan:

WARNING: ThreadSanitizer: data race (pid=17244)
  Read of size 8 at 0x7d480000b370 by main thread (mutexes: write M24165, write M969, read M24121):
    #0 Stream::getReadyQueueMemory() /home/daver/repos/couchbase/server/ep-engine/src/dcp-stream.cc:234 (ep.so+0x00000028f51e)
    #1 ActiveStream::addStats(void (*)(char const*, unsigned short, char const*, unsigned int, void const*), void const*) /home/daver/repos/couchbase/server/ep-engine/src/dcp-stream.cc:563 (ep.so+0x00000029452f)
    #2 DcpProducer::addStats(void (*)(char const*, unsigned short, char const*, unsigned int, void const*), void const*) /home/daver/repos/couchbase/server/ep-engine/src/dcp-producer.cc:551 (ep.so+0x00000027f1a0)
    #3 ConnStatBuilder::operator()(SingleThreadedRCPtr<ConnHandler>&) /home/daver/repos/couchbase/server/ep-engine/src/ep_engine.cc:3696 (ep.so+0x000000182d54)

  Previous write of size 8 at 0x7d480000b370 by thread T16 (mutexes: write M24143):
    #0 Stream::pushToReadyQ(DcpResponse*) /home/daver/repos/couchbase/server/ep-engine/src/dcp-stream.cc:211 (ep.so+0x00000028f4a6)
    #1 ActiveStream::backfillReceived(Item*) /home/daver/repos/couchbase/server/ep-engine/src/dcp-stream.cc:407 (ep.so+0x00000028d6e5)
    #2 CacheCallback::callback(CacheLookup&) /home/daver/repos/couchbase/server/ep-engine/src/dcp-stream.cc:87 (ep.so+0x00000028d4b3)
    #3 CouchKVStore::recordDbDump(_db*, _docinfo*, void*) /home/daver/repos/couchbase/server/ep-engine/src/couch-kvstore/couch-kvstore.cc:1563 (ep.so+0x00000031dec5)

See also: http://review.couchbase.org/54314 which originally fixed
this issue in watson; however it also fixed a couple of other issues
in the same patch.

Change-Id: Iae6a34403394e54c9d7213a7c2703be761e7dc0f
Reviewed-on: http://review.couchbase.org/62972
Well-Formed: buildbot <build@couchbase.com>
Reviewed-by: Will Gardner <will.gardner@couchbase.com>
Tested-by: buildbot <build@couchbase.com>