ensure we don't loop trying to write to a channel thats not connected (fix 100% CPU)

[djay]

fixes #418

This fixes a race condition where a quick forced socket close will make getpeername to fail and connected == False. If the request then has an error close_after_flush is set and this will result in a 100% CPU loop. This wiil ensure the channel is still closed in this case.

TODO:

• [x] add some more specific tests for double close
• [x] test for closing a error request thats not connected
• [x] test for maintainance not cleaning up an unconnected task
• [ ] change the fix to only prevent write specifically when already closed, not when not connected
• [ ] undo fix that closes if getpeername fails.

[digitalresistor]

Closing this since #435 was merged. Thank you!