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

gui/default/syncthing/settings/settingsModalView.html

@@ -15,8 +15,8 @@
             </div>
 
             <div class="form-group">
-              <label translate for="ListenAddressStr">Sync Protocol Listen Addresses</label>
-              <input id="ListenAddressStr" class="form-control" type="text" ng-model="tmpOptions._listenAddressStr">
+              <label translate for="ListenAddressesStr">Sync Protocol Listen Addresses</label>
+              <input id="ListenAddressesStr" class="form-control" type="text" ng-model="tmpOptions._listenAddressesStr">
             </div>
 
             <div class="form-group" ng-class="{'has-error': settingsEditor.MaxRecvKbps.$invalid && settingsEditor.MaxRecvKbps.$dirty}">
@@ -56,20 +56,6 @@
               </div>
             </div>
 
-            <div class="form-group">
-              <div class="checkbox">
-                <label>
-                  <input id="RelaysEnabled" type="checkbox" ng-model="tmpOptions.relaysEnabled"> <span translate>Enable Relaying</span>
-                </label>
-              </div>
-            </div>
-
-            <div class="clearfix"></div>
-            <div class="form-group">
-              <label translate for="RelayServersStr">Relay Servers</label>
-              <input ng-disabled="!tmpOptions.relaysEnabled" id="RelayServersStr" class="form-control" type="text" ng-model="tmpOptions._relayServersStr">
-            </div>
-
             <div class="form-group">
               <div class="checkbox">
                 <label>