Reimplement KeepaliveTimer using asynctimeout

[yschimke]

Current approach is awkward to cancel, would be cleaner to use AsyncTimeout.

  timeout_ = folly::AsyncTimeout::schedule(	
      std::chrono::milliseconds(heartbeatIntervalMs_),	
      eventBase_,	
      [this]() noexcept { heartbeat(); });
...
timeout_->scheduleTimeout(heartbeatIntervalMs_);