Fix QueryPage transaction plan

Fix QueryPage transaction plan

The COMMIT in updateSpecialPages.php was unmatched and just logged an
error. We would like the stale results to still be presented to the user
during the hours of query execution time, so doing a DELETE in
autocommit mode before the main query starts does not seem appropriate.
And holding a master transaction open for hours, with a lock on
querycache, is certainly not the right way to do it.

So, move the DELETE to after the completion of the main query, and wrap
a transaction around the updates to querycache and querycache_info so
that the user always sees a consistent populated UI. Remove the
unmatched COMMIT from updateSpecialPages.php.

Change-Id: I27c22b96f43a1064eb17a0c6a1c56d1f4a2dff9a