Misc follow-ups to I2fc3966e (a161c5e)

- Also add LinkCache::addLinkObj() calls to isRedirect(), getLength() and getContentModel()
- Changed error messages when LinkCache returns null, since this really should not happen anymore
- Removed incorrect comments
- Only use the value of the member variable if GAID_FOR_UPDATE is not passed
  (for consistency between the three methods)
- Mark tests that need database access as such due to the call to Title::getContentModel()
  (generally called through Parser::getFunctionLang())

Change-Id: I84e0c47cdf7412f2b7fa5f296d066b64b7bbfd42
Merge "Misc follow-ups to I2fc3966e (a161c5e)"