Ensure comments actually update in a transaction log

[pouneh]

Motivation

When a field is provided in an API, we expect that field to be leveraged. Specifically, in this case, the submit_transaction API wasn't using the provided comment to update the comment field in an existing transaction log.

In this PR

We update the comment both when signing the transaction, and when submitting it.

Test Plan

To run the rust test suite (including the test added in this PR), copy the consensus_enclave.css file to your current working directory, and then run tools/test.sh

Manually, you can exercise the scenario by doing the following

1. Create a transaction with the build_transaction API
2. Grab the transaction_log_id from the response
3. Check the transaction log with the get_transaction_log API using the transaction_log_id from Step 2
4. Observe that the comment field in get_transaction_log's response is an empty string
5. Submit the transaction using the submit_transaction API, and specify the optional parameter comment to be a string of your choice
6. Check the transaction log with the get_transaction_log API using the transaction_log_id from Step 2 and observe your new comment!