rewrote polling logic for injector.
It should now (when FDs are provided for incoming traffic) poll those along with any pending writes.
It will buffer up to one packet per endpoint, any more sent are sent back via Injector::full_pipe and it can decide what to do with them.
Still to do: when FDs are not provided, we need to create a thread and a mq, and use it to feed the listener thread instead of get_packet