Reload undefined EXT-X-PLAYLIST-TYPE based on Expires header

[robwalch]

Implement reload rules based on EXT-X-PLAYLIST-TYPE and Expires header:

• "VOD" immutable
• "Event" start immutable and end immutable after ENDLIST
• undefined elements can be removed from the start even after ENDLIST

The HLS spec says to use "Expires" headers to determine when/if to reload media playlists with an EXT-X-ENDLIST tag that is does not have a EXT-X-PLAYLIST-TYPE value of VOD.