Database: custom parser cache

[lahwaacz]

• [x] recomputable tables from MediaWiki (info only for the latest revisions)
• [x] implement a function to recursively expand templates transcluded on a page
• [x] parse latest revisions and cache data in these tables:
  • [x] pagelinks
  • [x] templatelinks
  • [x] imagelinks
  • [x] categorylinks
  • [x] langlinks
  • [x] iwlinks
  • [x] externallinks
  • category (discarded - tracks only the number of category members, which can be found online from joins)
  • [x] redirect
• [x] recursive per-page invalidation
• [x] implement select queries for these tables
  • [x] pagelinks (prop=links, prop=linkshere)
  • [x] templatelinks (prop=templates, prop=transcludedin)
  • [x] imagelinks (prop=images)
  • [x] categorylinks (prop=categories)
  • [x] langlinks (prop=langlinks)
  • [x] iwlinks (prop=iwlinks)
  • [x] externallinks (prop=extlinks)
  • [x] redirect (prop=redirects)
• [x] cache custom info specific to wiki-scripts:
  • [x] section headings (useful for e.g. link-checker.py)
  • follow-up in #36