Fix #1551: Backchannel failures in tests #1557
Fix various bugs around handling of disconnect in JsonIOStream and JsonMessageChannel.
Fix handling of DAP "terminated" event in debug.Session.
Add --ptvsd-logs and --pydevd-logs switches to pytest.
Improve message logging to fully capture the raw message data in the logs if deserialization fails.
Log all debuggee environment variables in debug.Session, and improve log readability.