fix compilation error in executorthread.cc

Atomic variables cannot be read directly without load()

Change-Id: If7a75a363f1096b0e6e5cfeb7bc7d4a07fb48433
Reviewed-on: http://review.couchbase.org/43144
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>