MB-19220: Ensure HashTable::size is atomic

MB-19220: Ensure HashTable::size is atomic

Fix data race as reported by ThreadSanitizer. I'm pretty sure this is
beniegn, as size can only be modified if *all* HashTable mutexes are
acquired (see HashTable::resize()), and all callers of
getBucketForHash() either do so with at least 1 HashTable mutex
acquired, or they perform double-checked locking (as per this
instance).

TSan report:
    WARNING: ThreadSanitizer: data race (pid=11329)
      Write of size 8 at 0x7ffe3a0ef1f8 by thread T9 (mutexes: write M45069, write M45070, write M45071):
        #0 HashTable::resize(unsigned long) ep-engine/src/stored-value.cc:370 (ep-engine_hash_table_test+0x00000050173c)
        #1 AccessGenerator::resize() ep-engine/tests/module_tests/hash_table_test.cc:314 (ep-engine_hash_table_test+0x0000004df4d9)
        #2 AccessGenerator::operator()() ep-engine/tests/module_tests/hash_table_test.cc:304 (ep-engine_hash_table_test+0x0000004df3fd)
        #3 SyncTestThread<bool>::run() ep-engine/tests/module_tests/threadtests.h:94 (ep-engine_hash_table_test+0x0000004e6bd8)
        #4 _ZL23launch_sync_test_threadIbEvPv ep-engine/tests/module_tests/threadtests.h:66 (ep-engine_hash_table_test+0x0000004c9ca8)
        #5 platform_thread_wrap platform/src/cb_pthreads.c:19 (libplatform.so.0.1.0+0x0000000035dc)

      Previous read of size 8 at 0x7ffe3a0ef1f8 by thread T8:
        #0 HashTable::getBucketForHash(int) ep-engine/src/stored-value.h:1470 (ep-engine_hash_table_test+0x0000004d32b6)
        #1 HashTable::getLockedBucket(int, int*) ep-engine/src/stored-value.h:1265 (ep-engine_hash_table_test+0x0000004d2e69)
        #2 HashTable::getLockedBucket(std::string const&, int*) ep-engine/src/stored-value.h:1295 (ep-engine_hash_table_test+0x0000004d277b)
        #3 HashTable::del(std::string const&) ep-engine/src/stored-value.h:1370 (ep-engine_hash_table_test+0x0000004d8f88)
        #4 AccessGenerator::operator()() ep-engine/tests/module_tests/hash_table_test.cc:306 (ep-engine_hash_table_test+0x0000004df430)
        #5 SyncTestThread<bool>::run() ep-engine/tests/module_tests/threadtests.h:94 (ep-engine_hash_table_test+0x0000004e6bd8)
        #6 _ZL23launch_sync_test_threadIbEvPv ep-engine/tests/module_tests/threadtests.h:66 (ep-engine_hash_table_test+0x0000004c9ca8)
        #7 platform_thread_wrap platform/src/cb_pthreads.c:19 (libplatform.so.0.1.0+0x0000000035dc)

Change-Id: I97c189310aafa8a002299f73cab9cbfb0e619768
Reviewed-on: http://review.couchbase.org/62910
Well-Formed: buildbot <build@couchbase.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Will Gardner <will.gardner@couchbase.com>