feat: simplified FIFO mempool implementation

[Milosevic02]

Heavy WIP – major parts still under development, not ready for review or merge.

Based on the issues raised in #1830, especially the unordered execution of transactions and inefficient handling of gas limits in the current CListMempool, I started working on a completely new mempool implementation from scratch.

The main problems with the existing solution:

• Transactions are not guaranteed to be returned in order of account sequence, which leads to execution failures (e.g. deploying a Realm before the account is created).
• The block assembly logic is based on declared gas, not actual gas used, which allows potential denial-of-service vectors and inefficient block space utilization.

After discussions and consultation with @zivkovicmilos, we concluded that these are core design flaws that cannot be resolved by simple tweaks. A ground-up rewrite is necessary.

I'm currently actively working on this implementation under Milos’s guidance, and the goal is to gradually build a more robust and production-ready mempool module.

[Gno2D2]

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 69.46903% with 69 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
tm2/pkg/bft/my_mempool/mempool.go	70.08%	55 Missing and 12 partials :warning:
tm2/pkg/bft/appconn/app_conn.go	0.00%	2 Missing :warning:

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[github-actions[bot]]

This PR is stale because it has been open 3 months with no activity. Remove stale label or comment or this will be closed in 3 months.

[zivkovicmilos]

@jefft0 It's not stale, it's a PR I have on my back burner 🙏

[jefft0]

@jefft0 It's not stale, it's a PR I have on my back burner 🙏

No problem. It was already stale for 53 days until the force push to master reset all the activity counters and Stale labels. That's why I restored the Stale labels. Nothing about this specific PR.