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/folder_recvonly_test.go

@@ -537,7 +537,8 @@ func setupROFolder(t *testing.T) (*testModel, *receiveOnlyFolder, context.Cancel
 
 	m.fmut.RLock()
 	defer m.fmut.RUnlock()
-	f := m.folderRunners["ro"].(*receiveOnlyFolder)
+	r, _ := m.folderRunners.Get("ro")
+	f := r.(*receiveOnlyFolder)
 
 	return m, f, cancel
 }