Reduced the DOS potential of 404 page floods

Reduced the DOS potential of 404 page floods

* showMissingArticle() will now only show the log excerpt
  if the page was deleted very recently or if the viewing
  user is logged in. Crawlers and poorly written bots tend
  to be logged out, and logged in users are better tracked.
  If many hits to the same recently deleted page are made,
  then the innoDB buffer pool should actually have the
  relevant index pages in cache (especially with query grouping).
* There have been several outages or DB performance problems
  caused by crawlers or sloppy mirrors hitting many bogus pages
  (due to bad URL generation, ect...). Previously a redis bloom
  filter was used to handle this, but was removed due to high
  complexity (especially if it was to be supported in multi-DC
  setup). This is a simpler solution to keep 404 pages cheap,
  as they cannot really be cached (there are innumerable possible
  titles that never existed).

Change-Id: If948602a32deb16dba21d232d0c6128568a980d6