Each batch of feeds sent to the worker pool is now guaranteed to contain unique feed URLs.
When `POLLING_LIMIT_PER_HOST` is set, an additional limit is applied to the number of concurrent requests per hostname, helping to prevent overloading a single server.
Note: Additional requests may still be made during feed refresh. For example, to fetch feed icons or when the web scraper is enabled for a particular feed.
- Replace a call to fmt.Sprintf with a concatenation
- Explicit declaration of return values in FetchJobs
- Initialize the size of FetchJobs return value to b.limit: when b.limit is
used, which is most of the time, this avoid resizing the slice, and when it
isn't, the size of the map is set to 0, which is equivalent to the previous
situation anyway.
- Move a call to `request.UserID(r)` to a lower scope.
