`fatxpool`: Master issue for upcoming work

[michalkucharczyk]

Following issues are planned for fork-aware transaction pool (follow-up of #4639).

### Tasks
- [ ] #5473
- [ ] #5497
- [ ] #5476
- [ ] #5493
- [ ] #5479
- [ ] #5626
- [ ] https://github.com/paritytech/polkadot-sdk/issues/5809
- [ ] #5483
- [ ] #5482
- [ ] https://github.com/paritytech/polkadot-sdk/issues/5978
- [ ] https://github.com/paritytech/polkadot-sdk/issues/6008
- [ ] https://github.com/paritytech/polkadot-sdk/issues/6070
- [ ] https://github.com/paritytech/polkadot-sdk/issues/6409

### Known bugs
- [ ] #5494
- [ ] #5487
- [ ] #5486

### Improvements
- [ ] #5474
- [ ] #5491
- [ ] #5495
- [ ] #5492
- [ ] #5490
- [ ] #5489
- [ ] #5496
- [ ] #5485
- [ ] https://github.com/paritytech/polkadot-sdk/issues/6056

### Add missing unit tests
- [ ] #5475
- [ ] #5478
- [ ] #5477
- [ ] #5480
- [ ] #5481
- [ ] #5488
- [ ] #5484

And the following improvements are required in transaction test tool:

### `ttxt` tasks
- [ ] https://github.com/michalkucharczyk/tx-test-tool/issues/9
- [ ] https://github.com/michalkucharczyk/tx-test-tool/issues/10
- [ ] https://github.com/michalkucharczyk/tx-test-tool/issues/11
- [ ] https://github.com/michalkucharczyk/tx-test-tool/issues/12
- [ ] https://github.com/michalkucharczyk/tx-test-tool/issues/13
- [ ] https://github.com/michalkucharczyk/tx-test-tool/issues/14
- [ ] https://github.com/michalkucharczyk/tx-test-tool/issues/15
- [ ] https://github.com/michalkucharczyk/tx-test-tool/issues/16

[Polkadot-Forum]

This issue has been mentioned on Polkadot Forum. There might be relevant details there:

https://forum.polkadot.network/t/the-fork-aware-transaction-pool/10468/1