ovsdb: Introduce for_each_txn_row() iterator function for transactions.

ovsdb: Introduce for_each_txn_row() iterator function for transactions.

A number of places in the transaction code want to iterate over all of
the txn_rows and possibly modify them.  It is getting messy to duplicate
the code to do this everywhere.  This commit introduces a new function that
encapsulates the iteration logic, efficiently handling modifications made
by the callback function.

Upcoming commits will add more uses of this iterator function.