lib/connections: Refactor

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

lib/discover/discover.go

@@ -15,15 +15,14 @@ import (
 
 // A Finder provides lookup services of some kind.
 type Finder interface {
-	Lookup(deviceID protocol.DeviceID) (direct []string, relays []Relay, err error)
+	Lookup(deviceID protocol.DeviceID) (address []string, err error)
 	Error() error
 	String() string
 	Cache() map[protocol.DeviceID]CacheEntry
 }
 
 type CacheEntry struct {
-	Direct     []string  `json:"direct"`
-	Relays     []Relay   `json:"relays"`
+	Addresses  []string  `json:"addresses"`
 	when       time.Time // When did we get the result
 	found      bool      // Is it a success (cacheTime applies) or a failure (negCacheTime applies)?
 	validUntil time.Time // Validity time, overrides normal calculation
@@ -41,12 +40,6 @@ type FinderMux interface {
 	ChildStatus() map[string]error
 }
 
-// The RelayStatusProvider answers questions about current relay status.
-type RelayStatusProvider interface {
-	Relays() []string
-	RelayStatus(uri string) (time.Duration, bool)
-}
-
 // The AddressLister answers questions about what addresses we are listening
 // on.
 type AddressLister interface {