Column group overwrite config

@michaelcahill, with this change, I can remove the code that sets the overwrite configuration for the column groups to false, and the test suite passes.

The required change is because the table WT_CURSOR.insert code was relying on a duplicate-key error from the primary cursor to decide if the operation was an insert or an update so that it could remove the index entries in the case of an update.   I've added an additional primary search in the WT_CURSOR.insert case since we can no longer rely on that error.

This change fixes #614.