perf(LocalFeedRepository): speed up local feed extraction

[FineFindus]

Speeds up the local feed extraction, by fetching more feeds concurrently and only applying a large timeout after fetching the full channels. This is based on the idea that YouTube's RSS feeds (extracted by the generic FeedInfo) have no/much less rate-limiting, likely due to the nature of RSS readers. I couldn't find an explicit statement from YouTube about that, but it makes sense. I've also increased to amount of concurrently fetched feeds in my fork for the last couple of months and had no noticeable impact so far.
A small timeout after each chunk is still applied, to be a good citizen of the web and not overwhelm/spam the server.

The used values may need to be adjusted in the future, either to allow for even faster extraction or throttling if a rate-limit is introduced.

[NeeRaj-2401]

I was just thinking that instead of waiting until all the sub-channels are extracted before updating the feed, we could update it as soon as new content is extracted from a channel. This would make the user feel that the feed is being updated more quickly, enhancing the overall UX.

[FineFindus]

While I agree that immediately showing new items is a good idea, I doubt a good implementation is possible, as we don't know which channels have uploaded (and in which order), so the new entries would constantly be reshuffled, which I think would be rather confusing and possibly nauseating. Additionally, constantly adding new items at (essentially) random points in the recycler view might lead to performance problems.

That aside, I don't see how this is related to this PR? I think it would be better to open a new issue.