Optimize the case of two external links that differ only in anchors

[Martoon-00]

Clarification and motivation

We do not check anchors of external links, so if two links differ only in anchor, we could consider them as "same" and perform only one poll for both.

Note that we already have a mechanism for avoiding two requests for two links that are exactly the same, and now we should carefully extend that; carefully because we rely on "same" notion to designate the same thing in several places. Maybe introduce one more newtype-wrapper, this type for external links.

Acceptance criteria

• For several external links that differ only in anchor we perform one request.
• We've considered which other parts of the URI are irrelevant for our checker, and got transparent over them too.