fix: Fix handling of disconnection events in the adapter

[simon-lemay-unity]

MTT-2467

How the adapter handled Disconnect events from the transport needed to be fixed in two ways:

1. Send and receive queues were not cleared, meaning memory would stay allocated for nothing.
2. The m_State would be set to Disconnected, but that should only be done on clients. Hosts have their m_State set to Listening and it should stay that way after a client has disconnected. This is likely what was causing MTT-2467.

This PR also brings in changes from PR #1649 since it provides a clean way of clearing the send queues. I also modified the connection tests to improve their stability (waiting for the client to be connected since this guarantees the server also is, otherwise there could be timing issues where the server is connected but not the client).

Changelog

com.unity.netcode.adapter.utp

• Fixed: Fixed and issue where a server would fail to disconnect a client if another client had previously disconnected itself.

Testing and Documentation

• Includes unit/integration tests.
• No documentation changes or additions were necessary.

[ashwinimurt]

cc @zain-mecklai