There is a bug in the scenario of the following sequence of events:

- `call.start()` 
- received retryable status and about to retry
- The retry attempt Substream is created but not yet `drain()` 
- `call.cancel()`

But `stream.cancel()` cannot be called prior to `stream.start()`, otherwise retry will cause a failure with IllegalStateException. The current RetriableStream code must be fixed to not cancel a stream until `start()` is called in `drain()`.

…#8393)

While adding regression tests to grpc#8386, I found a bug in an edge case: while retry attempt is draining the last buffered entry, if it is in the mean time committed and then we cancel the call, the stream will never be cancelled. See the regression test case `commitAndCancelWhileDraining()`.