MB-19249: Address possible data races in ConnHandler context

MB-19249: Address possible data races in ConnHandler context

WARNING: ThreadSanitizer: data race (pid=2443)

  Read of size 1 at 0x7d5000016a58 by thread T10:
    #0 ConnHandler::doDisconnect() /home/abhinav/couchbase/ep-engine/src/tapconnection.h:375 (ep.so+0x000000058416)
    #1 ExecutorThread::run() /home/abhinav/couchbase/ep-engine/src/executorthread.cc:112 (ep.so+0x0000000f87f6)
    #2 launch_executor_thread(void*) /home/abhinav/couchbase/ep-engine/src/executorthread.cc:33 (ep.so+0x0000000f8395)
    #3 platform_thread_wrap /home/abhinav/couchbase/platform/src/cb_pthreads.c:23 (libplatform.so.0.1.0+0x000000003d31)

  Previous write of size 1 at 0x7d5000016a58 by main thread:
    [failed to restore the stack]

  Location is heap block of size 480 at 0x7d5000016a00 allocated by main thread:
    #0 operator new(unsigned long) <null>:0 (engine_testapp+0x00000005084d)
    #1 DcpConnMap::newConsumer(void const*, std::string const&) /home/abhinav/couchbase/ep-engine/src/connmap.cc:969 (ep.so+0x000000048384)
    #2 EventuallyPersistentEngine::dcpOpen(void const*, unsigned int, unsigned int, unsigned int, void*, unsigned short) /home/abhinav/couchbase/ep-engine/src/ep_engine.cc:6189 (ep.so+0x0000000d3668)
    #3 EvpDcpOpen(engine_interface*, void const*, unsigned int, unsigned int, unsigned int, void*, unsigned short) /home/abhinav/couchbase/ep-engine/src/ep_engine.cc:1494 (ep.so+0x0000000b4e5f)
    #4 mock_dcp_open(engine_interface*, void const*, unsigned int, unsigned int, unsigned int, void*, unsigned short) /home/abhinav/couchbase/memcached/programs/engine_testapp/engine_testapp.cc:488 (engine_testapp+0x0000000bb015)
    #5 test_dcp_consumer_flow_control_aggressive(engine_interface*, engine_interface_v1*) /home/abhinav/couchbase/ep-engine/tests/ep_testsuite.cc:3826 (ep_testsuite.so+0x00000006ecfd)
    #6 execute_test(test, char const*, char const*) /home/abhinav/couchbase/memcached/programs/engine_testapp/engine_testapp.cc:1090 (engine_testapp+0x0000000b946c)
    #7 __libc_start_main /build/buildd/eglibc-2.19/csu/libc-start.c:287 (libc.so.6+0x000000021ec4)

Change-Id: Id5223e93c416e5e5287c137d561aea1e453cbd41
Reviewed-on: http://review.couchbase.org/62970
Well-Formed: buildbot <build@couchbase.com>
Reviewed-by: Will Gardner <will.gardner@couchbase.com>
Tested-by: buildbot <build@couchbase.com>