feat: store first_block on transactions

[andreabadesso]

This PR handles first_block changes (by storing them in the transaction table) and also handles multiple changes in a transaction metadata in the same event

Motivation

After adding the first_block handling mechanism, we stopped handling nc_execution changes, upon investigation I've noticed that we were expecting one METADATA_CHANGED event for each change which is not what actually happens -- we receive a single metadata changed event with all changes to metadata

Acceptance Criteria

• metadataDiff detects all independent metadata changes in a single event and returns them as an array
• A dispatch queue processes each change one-by-one before returning to idle
• handleNcExecVoided no longer needs to know about first_block — each handler has a single responsibility
• first_block is now a transaction column and gets handled by the new metadata diff

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged
• Make sure either the unit tests and/or the QA tests are capable of testing the new features
• Make sure you do not include new dependencies in the project unless strictly necessary and do not include dev-dependencies as production ones. More dependencies increase the possibility of one of them being hijacked and affecting us.

[andreabadesso]

Thanks Claude