This sacrifices the ability to return an error when creating the service
for being more persistent in keeping it running.
GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3270
LGTM: AudriusButkevicius, canton7
1. Removes separate relay lists and relay clients/services, just makes it a listen address
2. Easier plugging-in of other transports
3. Allows "hot" disabling and enabling NAT services
4. Allows "hot" listen address changes
5. Changes listen address list with a preferable "default" value just like for discovery
6. Debounces global discovery announcements as external addresses change (which it might alot upon starting)
7. Stops this whole "pick other peers relay by latency". This information is no longer available,
but I don't think it matters as most of the time other peer only has one relay.
8. Rename ListenAddress to ListenAddresses, as well as in javascript land.
9. Stop serializing deprecated values to JSON
GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/2982
This fixes both a race condition where we could assign s.stop from one
goroutine and then read it from another without locking, and handles the
fact that listener may be nil at shutdown if we've had a bad
CommitConfiguration call in the meantime.
