Use the file position stored in the transaction_id whenever possible.
It is possible when _check_txn_pos() returns true; i.e. when the file
position does point to the transaction record header.

Remove ostloc and here arguments to _txn_undo_write(), as they can be
computed after the call.

Omit redundant tests in _txn_undo_write().  All the paths that lead
here verify that the file position is valid.

Remove some attribute lookup optimizations.