chore(sequencer): refactor app

[ethanoroshiba]

Summary

Refactored <prepare/process>_proposal_execute_transactions methods.

Background

These methods previously contained a lot of shared code. This is meant to eliminate this code duplication and also make the steps that are happening in the proposals clearer and easier to understand.

Changes

• Renamed prepare_proposal_execute_transactions to prepare_proposal_tx_execution to better represent that this is the execution step of the larger prepare_proposal method.
• Renamed process_proposal_execute_transactions to process_proposal_tx_execution.
• Refactored block size checks and tx execution to shared helper functions, which take a new enum Proposal containing information necessary for either Prepare or Process.

Testing

Passing all tests.

Changelogs

No updates required

Related Issues

closes #1785

[Fraser999]

Generally looks good. A couple of minor suggestions though.