Updates after performance review

* Use MapCacheLRU in displayGlobalPage()
* Optimize brokenLink to avoid DB queries if already known
* Don't check prefs if GlobalPreferences is disabled
* Use MapCacheLRU in getCentralTouched() and call
  reuseConnection()
* getRemoteParsedText() has a fixed length memcached key
* Optionally use $wgConf in getRemoteURL()
* Use a custom timeout for internal API requests
* Gracefully fail if an internal API request fails

Bug: 70585
Change-Id: I7440151f29bf38b6c135d8508ac1a0cf24a5677e