Audit GrpcFuture cancellation

GrpcFuture is currently using an `interruptTask()` override to cancel the RPC. This doesn't appear to be correct, as it is only called when `cancel(true)` is invoked on the outer Future. There are a few problems with this:

1. The Future's cancellation is not atomically set with the calls. If any callbacks start while the future is still cancelling, they may see the call is not yet closed.
2. The call is only cancelled when true is set. This appears to be an incorrect interpretation of `mayInterruptIfRunning`, which is intended for tasks on an ExecutorService. This is also different than internal RPC implementations
3. interruptTask() is bordering on being deprecated, and may not be available in the future
4. Futures can be cancelled from any thread, but ClientCall.cancel is not thread safe.

cc: @lukesandberg