util: Porting NotificationSubscriber, FaceMonitor, and NotificationStream from NFD

New generic classes:
- `util::NotificationSubscriber` (`util/notification-subscriber.hpp`)
- `util::NotificationStream` (`util/notification-stream.hpp`)

NFD Face monitoring class:
- `nfd::FaceMonitor` (`management/nfd-face-monitor.hpp`)

Change-Id: I2ab0a2cd9d7e3ac07036f290f0b4de5eb16e6e38