[Performance] Improve digests/sec indication

Don't let past digest counts bias current rate shown;
https://github.com/nasa/openmctweb/issues/369

example/profiling/src/DigestIndicator.js

@@ -39,8 +39,11 @@ define(
                 start = Date.now();
 
             function update() {
-                var secs = (Date.now() - start) / 1000;
+                var now = Date.now(),
+                    secs = (now - start) / 1000;
                 displayed = Math.round(digests / secs);
+                start = now;
+                digests = 0;
             }
 
             function increment() {