Request LL-HLS playlists with _HLS_part param starting at 0 for next sn

[robwalch]

This PR will...

• Request LL-HLS playlists with the _HLS_part param set to 0 and _HLS_msn incremented when the last sn matches the segment of the last part.

Why is this Pull Request needed?

The current implementation always requests the last part index + 1, which depends on servers responding with part 0 as per this entry in https://tools.ietf.org/html/draft-pantos-hls-rfc8216bis-08#section-4.4.6.2

If the Client requests a Part Index greater than that of the final
Partial Segment of the Parent Segment, the Server MUST treat the
request as one for Part Index 0 of the following Parent Segment.

The downside is that also results in additional caching on the edge since you could have requests for something like _HLS_msn=1&_HLS_part=4 which must return the same thing as _HLS_msn=2&_HLS_part=0. By detecting that the last part and last segment match, we know there are no more parts for the current sn, and can increment sn and reset _HLS_part to 0 resolving #3571 and #3771.

Resolves issues:

Resolves #3571 #3771

Checklist

• changes have been done against master branch, and PR does not conflict