Use performance.now to limit load queue processing time.

Previously we were using Date.now, but its resolution is terrible, at
least on Windows.  Cesium is only supposed to spend up to 5ms per frame
processing the tile load queue, but WTF reported it frequently taking over
10ms.  After this change, processing time is actually limited to ~5 ms,
which reduces frame rate drops while loading tiles.
Merge remote-tracking branch 'origin/performanceNow' into terrainDebug