Improve efficiency of metric storage

Created by: LloydW93

dash.js currently stores a variety of metrics, such as TCP info, HTTP requests, bitrate changes, buffer level, dropped frames, manifest updates...

Some of these metrics are large objects, and some of them append data to the metrics store incredibly often.

To prevent long-running streams (for example, a screen displaying a 24x7 simulcast of a broadcast channel) from experiencing performance issues, dash.js should endeavour to minimise the amount of data it keeps in its metrics store.