Rolled back locks may cause collisions

While working on #917 I realized that rolled back locks may unnecessarily cause collisions. If a transaction could only lock a subset of columns, then it will fail and rollback the locks it did acquire. These rolled back locks should not cause other transactions to fail. It seems that the code may treat these rolled back locks as successful locks which can cause other transaction to fail unnecessarily.

The issue names no files or tests; begin by reading #917 and tracing transaction lock acquisition, partial-lock failure, and rollback behavior. Confirm how rolled-back locks are represented and whether other transactions can observe them as successful. Done means locks acquired before a failed subset request no longer cause unrelated transactions to fail, with regression coverage for that scenario.

Written by the indexing model from the issue text.