Feed reader should globally limit how many requests are sent to a given domain

Taken from https://github.com/matrix-org/go-neb/issues/382

Basically, we don't want to register 50000 feeds against matrix.org, and then have the RSS system hit it 50000 times in a cycle. Better to limit on domain if possible.

Start by locating the feed reader and the code that schedules or sends feed requests; the issue does not name files or tests. Trace how multiple registered feeds are processed, then determine how a per-domain request limit would be represented and verified. Done means feeds for one domain are globally capped rather than issuing one request per registered feed in a cycle.

Written by the indexing model from the issue text.