Make per-line flushing configurable in udp_bridge

See #155.
(also, since d3f8c792751578ecde0ff92b97acf75b093ae9b4, the UDP bridge seems to get killed off after 10 minutes, but may still hold unflushed data - on low traffic udp_bridges, flushing every line isn't an issue).

To enable, create a `flush()` function in the udp_bridge_conf that returns True.