Properly implement connection shutdowns in collection.rs

[tomaka]

Very work in progress

[tomaka]

While the changes in collection.rs are (tough but) mechanical, I'm unsure how to propagate these changes to peers.rs and service.rs. At the moment, they aren't capable of handling half-closed connections. In particular, service.rs automatically unallocates the peer's slot when its connection is shutting down.

Maybe doing #60 first is a better idea.

[tomaka]

This can now be worked upon again after #1200. However I'll most likely open a new PR.