SERVER-12380 Fix error that prevented UpdateDriver from being reused.

The UpdateDriver represents a document->document transformation described by the
update parameter of update-like database operations.  It could already be used
repeatedly on multiple documents, so long as it represented only a single transaction.
However, it was designed to also support reprogramming to handle different transformations.
A programming error prevented this, by not clearing a piece of old state when reparsing.