Move write-only transactions to the end of commit batches

Write-only transactions can't be aborted due to conflicts (barring self-conflicts), but they can cause unnecessary intra-batch conflicts for other transactions. This problem can be mitigated by moving all transactions with empty read-conflict sets (again, excluding self-conflict keys) to the end of all commit batches. The two main downsides to this approach are:

1) Increases proxy CPU usage.
2) Makes it more likely that transactions from the same client will be committed in a different order than `commit` is called. This is already a possibility though, so this is not a major weakness.

Given these weaknesses, this may only be a worthwhile change for certain workloads that have lots of write-only transactions and a high conflict-rate.

Start by tracing how the proxy constructs commit batches and represents read-conflict sets; the issue gives no file or test entry point. Determine where transactions with empty read-conflict sets can be identified while excluding self-conflict keys, then assess the CPU and commit-order tradeoffs. Done means the behavior is implemented and validated for conflict-heavy workloads without breaking transaction ordering guarantees.

Written by the indexing model from the issue text.