Implement faster EVENT style playlist parsing

[dcoufal]

Description

EVENT style playlists just add fragments onto the end of an existing variant.

This ticket is to implement a special parsing pathway that will take a previous EVENT style variant and update it with a new version. Only a small amount of parsing will have to be done to update the HLSPlaylist object.

Tasks

• [ ] Provide an "update" style method for HLSParser that will take an previously parsed HLSPlaylist as an argument and will parse this playlist specially to save effort.
• [ ] This new method should handle edge and error conditions well (i.e. both must be event style variants, should share a URL, difference between each should be small). If these conditions are not met, the new parse method should fall back to normal parsing.

[dcoufal]

Why is this useful?

EVENT style variants have a long list of fragments. They can only be added to the end of the playlist as time goes forward. Anything in the middle cannot be changed. This is according to the HLS spec (https://tools.ietf.org/html/draft-pantos-hls-rfc8216bis-03#section-6.2.1. The server MUST NOT change the Media Playlist file, except to: Append lines to it

So, if we have an hours long Event playlist, it's more efficient to just parse the last part of the playlist and add them to the already parsed playlist from the last update, if you are parsing HLS for playback.

This ticket is to implement such an update method. It's totally optional for users of mamba.

[dcoufal]

Fixed