Rework playlist-selectors

[forbesjo]

Description

Playlist selectors like https://github.com/videojs/http-streaming/blob/master/src/playlist-selectors.js#L260 are somewhat difficult to follow because they rely on properties from the source handler instance (https://github.com/videojs/http-streaming/blob/master/src/videojs-http-streaming.js#L404), the code can be more straightforward and easier to follow.