Speed up link checking

Right now link checking is one of the slowest parts of `./build_docs --all` because it checks all links from all books. I'd like to speed this up somehow. I suspect we could do something as simple as skipping checking the links for books that weren't modified and don't point to modified books. We may be able to do better and store the list of extracted links. That process may be faster to do once we've fully converted to Asciidoctor because we can do it on the asciidoc AST rather than the generated html.