lib/model: Refactor folderRunners to use a serviceMap (#9071)

Instead of separately tracking the token.

Also changes serviceMap to have a channel version of RemoveAndWait, so
that it's possible to do the removal under a lock but wait outside of
the lock. And changed where we do that in connection close, reversing
the change that happened when I added the serviceMap in 40b3b9ad1.

lib/model/service_map_test.go

@@ -105,10 +105,11 @@ func TestServiceMap(t *testing.T) {
 
 		// Remove two of them from within the iterator.
 
-		sm.Each(func(k string, v *dummyService) {
+		sm.Each(func(k string, v *dummyService) error {
 			if strings.HasPrefix(k, "remove") {
 				sm.RemoveAndWait(k, 0)
 			}
+			return nil
 		})
 
 		// They should have stopped.