Handle case where grpc backend may be deregistered from registry but is still handling traffic

[nathanejohnson]

This is an attempt to gracefully shut down a connection to a grpc backend that gets deregistered from a registry, such as consul. This is described as point 1 in #807 . I'm not sure how common this will be, mostly a deregister event is caused by a failing health check, but this is easy enough to implement and shouldn't break anything so worth trying