Fix a shutdown crash found by TSAN tests
Fix a shutdown crash found by TSAN tests

The crash happens when there are still pending callbacks in the gRPC
queue when shutting down. This happens when the request has been
finalized due to an error, but there was still a call to the backend in
flight. As there are no more requests NGINX thinks it's safe to
shutdown. Processing these events try to enqueue new events into the
gRPC completion queue, which causes the crash as the completion queue
has been shut down.

Change-Id: I210be89b1e59c6893a99989171e63c36a6f08135

cloudendpoints/esp,cloudendpoints/esp,cloudendpoints/esp,cloudendpoints/esp,cloudendpoints/esp,cloudendpoints/esp,cloudendpoints/esp,cloudendpoints/esp