[Proposal] [WIP] Improve multi-URL priorization

[peaBerberian]

Our work toward implementing ContentSteering (#1142) also brought as a side-effect a better CDN priorization algorithm, when multiple ones are available for a segment:
A request returning with an error will result with the corresponding CDN being temporarily "downgraded" for 60 seconds. The corresponding CDN would then be avoided when possible for this amount of time, regardless of the resource in question.

This seems to be very useful for all cases where multiple CDNs are available and not just when relying on Content Steering - which still seems to be in a draft phase. Consequently this PR removes all ContentSteering-related bits from that previous PR (it may be merged in the future) to only let the code:

• identifying a particular "CDN" when multiple ones are available, by relying in DASH on the corresponding absolute BaseURL
• prioritizing CDNs between one another: first by initially relying on the first one announced in the Manifest, considering the next URL only once an error has been encountered on the previous one - until its "downgrade" delay is expired (previously we tried each CDN in order per-segment, meaning that the problematic CDN would be retried on each segment request).