Non-singleton ManifestLoader causes problems with dash.js metrics

Created by: greg80303

If you pre-fetch licenses (using samples/dashif-reference-player/eme.html) for two pieces of content and then try to play them both, the second play will fail. This is due to the new way that we expose the manifest loading functionality via the MediaPlayer API. ManifestLoader is no longer a singleton since we use a new instance for loading each call to the MediaPlayer API as well as a new instance for each time the MediaPlayer is asked to play a URL. The objects that make up the metrics system in dash.js are singletons and this causes problems with they try to get data from manifest loads.