Repeated URI parsing slows 3D Tiles performance

[jjhembd]

In large tilesets, especially where many tiles are pointing to external tilesets, URI parsing can add a significant overhead to tile loading. Profiling on one of these tilesets shows more than 20% of JavaScript time being used by URI.parse and related functions.

There are two things we could do to improve:

• Store a parsed URI in the Resource class, so that frequently-used prototype methods like .clone, .getUrlComponent and .getDerivedResource don't have to re-parse it from a string every time.
• Add a .serverKey property to the Request constructor. This would reduce calls to RequestScheduler.getServerKey , which parses a URI from a string. Note: this property is already assumed in Cesium3DTile, where requestSingleContent sends a serverKey to the constructor—but the constructor doesn't use it.

Related: #11168. Ideally, any changes should be written in a way that will be easy to convert to native URL and URLSearchParams.