MB-19259: Fix data race on DcpConsumer::backoffs
MB-19259: Fix data race on DcpConsumer::backoffs

This member is read by stats processing concurrently with DcpConsumer
updating it. Change to RelaxedAtomic<>

As reported by ThreadSanitizer:

WARNING: ThreadSanitizer: data race (pid=24629)
  Read of size 4 at 0x7d5000016b9c by main thread (mutexes: write M27034, write M2479):
    #0 void ConnHandler::addStat<unsigned int>(), void const*) ep-engine/src/tapconnection.h:291:18 (ep.so+0x00000004e993)
    #1 DcpConsumer::addStats(), void const*) ep-engine/src/dcp/consumer.cc:731:5 (ep.so+0x00000005a2c3)
    ...

  Previous write of size 4 at 0x7d5000016b9c by thread T10:
    #0 DcpConsumer::processBufferedItems() ep-engine/src/dcp/consumer.cc:755:17 (ep.so+0x0000000539c3)
    #1 Processer::run() ep-engine/src/dcp/consumer.cc:57:13 (ep.so+0x00000005376b)
    #2 ExecutorThread::run() ep-engine/src/executorthread.cc:115:26 (ep.so+0x0000000e944e)
    ...

Change-Id: Iabddcc06213fbb80815d4b464c459adb922a0eff
Reviewed-on: http://review.couchbase.org/62977
Well-Formed: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Jim Walker <1cd02e31b43620d7c664e038ca42a060d61727b9@couchbase.com>
Reviewed-by: Will Gardner <cbd6f5d9092d9ef7d5c0510d53bba134b22c02f2@couchbase.com>
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

couchbase/ep-engine,daverigby/ep-engine,daverigby/kv_engine,membase/ep-engine,daverigby/ep-engine,daverigby/ep-engine,teligent-ru/ep-engine,membase/ep-engine,daverigby/ep-engine,teligent-ru/ep-engine,membase/ep-engine,teligent-ru/ep-engine,teligent-ru/ep-engine,membase/ep-engine,daverigby/kv_engine,couchbase/ep-engine,couchbase/ep-engine,daverigby/kv_engine,daverigby/kv_engine,couchbase/ep-engine