[lldb-dap] Test Gardening, improving DebugCommunication.

Improving the readability and correctness of DebugCommunication by adding type annotations to many parts of the library and trying to improve the implementation of a few key areas of the code to better handle correctness.

Specifically, this refactored the `DebugCommunication._handle_recv_packet` function to ensure consistency with the reader thread when handling state changes and improved the `DebugCommunication._recv_packet` helper to make it easier to follow by adding some additional helpers.

Author: ashgti