API performance enhancements (bug 13511):

• Replaced $wgAPIUCUserPrefixMinLength with the more generic $wgAPIMaxDBRows
• Added ApiBase::checkRowCount() which checks whether the amount of rows to be scanned is acceptable (i.e. <$wgAPIMaxDBRows). Not using this anywhere (yet?), but it's nice to have
• Killed a filesort in the usercontribs query, query is now indexed nicely
• Dropped the minimum length for ucuserprefix since it's no longer needed (query optimized)
• Removed drnamespace from list=deletedrevs (filesorts 8M rows for drnamespace=0)
• Support multiple orderings in ApiBase::addWhereRange()