Move listener code to separate origin-listener package

[franckc]

Currently, discovery includes the code for both the discovery server and the listener.
It would be cleaner to separate the listener code out. I'm thinking the structure would look like:

• origin-discovery: discovery server
• origin-listener: listener server
• origin-search: search specific code, used by both origin-listener and origin-discovery

[franckc]

This could be a good meaty issue for someone looking at getting familiar with our back-end.