Disconnect handler

[BHandle]

This modifies the disconnect behavior of a Client in the following ways:

• adds a WhenDisconnected event that allows the creator to react to the Server closing.
• KeepAlive pings are stopped, rather than only stopping if a disconnect message is sent to the Server.
• An Exception is prevented if Disconnect is called when the Connect did not succeed. (also required if the last bullet is accepted)
• A KeepAlive ping failure is caught and treated as a Server disconnect, rather than bubbling the exception on the Timer.

[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.