Database: close() should not commit transactions

Database: close() should not commit transactions

Transactional databases normally roll back when a connection is closed
with an open transaction rather than committing them, so MediaWiki
committing them is unexpected.

There are two cases being changed here: automatic transactions without
writes and manual transactions. For the former it shouldn't make a
difference if we commit or roll back since no writes were done anyway.
The latter has logged a message since MW 1.31 (I0992f9a8), and that
warning has not been logged in Wikimedia production in the past 60 days
so we should be ok there too.

Bug: T206147
Change-Id: Ieceef4deb49044db8f0622d38ee76c9d9f39704c