Wait for precondition checks before beginning the DB transaction.

Wait for precondition checks before beginning the DB transaction.

SQLA's DBSession implicitly starts a PG transaction. When this
happens before the precondition checks (blank game, verified req, etc)
then an unneccesary transaction is started. Delay grabbing a new
DBSession until AFTER these checks are done prevents these useless
transactions from occurring. This means we should see little to
no ROLLBACK transactions in Munin once this is in place.