Implements #134 source groups

Adds source groups to reconstruct using a new high precision timer abstraction. Gives an ability to precisely control latency and guarantee frame sync. Some work still required to better allow independence between receiving real frames and generate virtual viewpoints. Work also needed to remove callbacks for a clean shutdown.

There are many enhancements to network code efficiency also.

components/rgbd-sources/src/stereovideo.cpp

@@ -205,6 +205,9 @@ bool StereoVideoSource::compute(int n, int b) {
 		left_.download(rgb_, stream_);
 		stream_.waitForCompletion();  // TODO:(Nick) Move to getFrames
 	}
+
+	auto cb = host_->callback();
+	if (cb) cb(timestamp_, rgb_, depth_);
 	return true;
 }