[Hls-interest] Allow URI changes

[Philipp <[e-mail]>]

Hi

I currently write a HLS Loadbalancer. The idea is to change the URI of
the Media Segments, to select which client should load the Media Segments
from which server. This might lead to change the URI of Media Segments
for two causes:

1. Policy changes
It might be necesarry change the server of a client during runtime of
the stream. Yes the server could track the last segment number he
deliverd to a client. This might lead to playback issues, depending on
the reason the policy changed. Also this directly leads to the secound
cause.

2. Lost of client tracking
There are some edge cases the server don't recognice the client. In this
case the server might give the client an other URI for the same Media
Segment.

Unfortunately section 6.1.2 don't allow to change the URI of a Media
Segment and sections 6.3.4 say a client should halt playback if he
detects a changed URI.

I propose for the next version of the standard, to allow the server to
change the URI and force the client to handle a changed URI in a sane
way. So the client must not reload already loaded segments and must not
stop loading already started downloads, but should use the new URI for
new or restarted downloads.

Philipp