[k2] publish module

[neonphog]

The publish module on the send side should be a simple queue with basic timeout and limited parallelism logic to sequentially send hashes to remotes.

On the receive side it will just add those hashes to the fetch queue.

All other complexity will be deferred until performance testing shows it is required.

Try using the real production publish module as the implementation in the core crate. We can always move it out if it gets too complicated.

[jost-s]

Agent infos should be published too.

[matthme]

Completed with #119 and #124