protocol-native: rework resume of busy client

If a client becomes unbusy again, signal a resume event so that the
messages are processed in the next mainloop iteration. This gives the
current iteration time to perform cleanups if needed.

Remove the look hook and always do flushing with an IO_OUT event. Rework
some things so that we can flush right after processing input without
having to go through a loop iteration.

See #298