Fix schema generation for pointer variables, minor logging change

Hi -- two small changes.

First, a bug: all pointer types become varchar(255) in the generated table schema.

The latter is a minor logging improvement: we often open a transaction and defer tx.Rollback() -- but this always logs both the commit and rollback.  We take advantage of Commit()/Rollback()'s documented behavior (returning ErrTxDone once it is closed) to suppress the extra log.