perf: Reduce memory usage of `CommittedTransaction`

[dima74]

Context

Related: #5083

Solution

CommittedTransaction::error takes 200 bytes and is None in case of a successful transaction. Using Option<Box<...> reduces memory usage of iroha approximately by 10% (considering that all transactions are successful and with testing configuration as in #5083)

Checklist

• [ ] I've read CONTRIBUTING.md.
• [ ] (optional) I've written unit tests for the code changes.
• [ ] All review comments have been resolved.
• [ ] All CI checks pass.

[mversic]

relevant #5001