Data serialization fix

[BHandle]

Previously, the KeepAlive pings could cause serialization errors by injecting data in-between a write header and payload. This would cause the receiving reader to receive data that was prefixed by the Ping's size header and exception.

By ensuring that only one thread has access to the writer at a time, we can eliminate this risk and ensure the integrity of Header/Payload pairs.

I also changed the second size header byte to be a modulus operation. Even over millions of iterations, modern compiled modulus is still approximately the same performance. That said, however, there was nothing wrong with the previous calculation, I just find modulus to be more clear in intent.

[BHandle]

When creating this, I did not personally use the encrypted or stateful variants, and I haven't echoed any changes to those implementations. It's possible that these changes should also be performed there to keep each of them in sync.