strip: use the 'finally: tr.release' pattern during stripping The previous code, was calling 'abort' in all exception cases. This was wrong when an exception was raised by post-close callback on the transaction. Calling 'abort' on an already closed transaction resulted in a error, shadowing the original error. We now use the same pattern as everywhere else. 'tr.release()' will abort the transaction if we escape the scope without closing it. We add a test to make sure we do not regress.