ws: refactor, bug fixes, improvements (#155)

- split swarm worker into two modules
- split socket worker into two modules
- keep track of which offers peers have sent and only allow matching
answers
- always clean up after closing connection
- use channel for telling connections to close
- move some logic into new ConnectionRunner struct
- use slotmap for connection reference storage
- fix double counting of error responses
- actually close connections that take too long to send responses to
- remove announced_info_hashes entry on AnnounceEvent::Stopped

TODO.md

@@ -3,9 +3,6 @@
 ## High priority
 
 * aquatic_ws
-  * Store OfferId that a peer has sent out and only allow answers matching
-    them to be sent? HashMap<(OfferId, PeerId), ValidUntil> could work, but
-    not if peers reuse offer ids
   * Validate SDP data
 
 ## Medium priority