Make LinksUpdate run without a db transaction.

Make LinksUpdate run without a db transaction.

This causes LinksUpdate not to be wrapped by a transaction, fixing
a problem caused by the introduction of SqlDataUpdate. LinksUpdate
never used a transaction wrapping the entire update, but only small
transactions for some database operations. SqlDataUpdate however introduced
an implicite transaction bracket for the entire update, causing the old,
inner transactions to be nested transactions, which is unsupported and
may cause database corruption (because starting a "nested" transaction
will commit the previous transaction prematurely).

Once we have support for nested transactions, LinksUpdate may again use a
transaction bracket for the entire update, but this is also subject to
performance considerations.

Change-Id: I80faf2ed79b56a3990a1724516e65621ca5bbece