Make MutTxId::update really idempotent wrt. TxData

[Centril]

Description of Changes

Given:

insert(row);
commit();
update(row);

there is a bug in MutTxId::update where it doesn't elide the delete/insert pair, unlike MutTxId::insert.

This PR fixes the bug.

API and ABI breaking changes

Not yet.

Expected complexity level and risk

2, fairly local change.

Testing

The test_update_idempotent test is updated to assert the idempotence of TxData in this scenario.

[gefjon]

Code looks good, test looks good, and I tested locally and verified that I don't see the issue any more. Thanks so much for your quick work!