fix: eliminate possibility of unhandled socket exceptions

[gkc]

- What I did

• fix: eliminate possibility of unhandled socket exceptions
• Resolves #1689

- How I did it

• take up at_server_spec 5.0.0 which changes return type of AtConnection.write from void to Future
• change signature of write in all subclasses / implementers of AtConnection
• BaseSocketConnection.write now calls await socket.flush() after calling socket.write(...)
• ensure all code which calls .write uses await connection.write(...)
• InboundConnectionImpl and OutboundConnectionImpl now call socket.done.onError(...) to ensure that all async errors are handled appropriately

- How to verify it

• Automated tests pass
• gkc's manual testing process, whereas previously async socket exceptions were going to the runZonedGuarded error handler, now the exceptions are being handled correctly in the appropriate place