Tracking issue for codebase modularization and other dev velocity improvements

[robin-near]

General theme: split chain into logical components; have testing for each logical component.

Some items we have come up with so far (many of these need further brainstorming)

For those who have access: link to the VC where we discussed this issue

• [ ] Split ChainStore, so we can split Chain
• [ ] Add detailed documentation as comments on top of every Chain and Client function.
• [ ] Split Client
• [ ] Improve the composability of testing utils, such as TestEnv and the intimidating setup_mock_all_validators, so we can write more granular tests and be more precise about how tests are set up.
  • [ ] https://github.com/near/nearcore/issues/8190
• [ ] Export more debugging info for partial encoded chunks handling
• [ ] Have a clearer API for working with the chain topology (e.g. no "head", "header_head", "head_header" business); fix issues related to incorrect/inaccurate use of chain topology (#8154). Have a isolated component just to manage topology.
• [x] Finish ShardsManager refactoring to move it onto separate actix (#7662)
• [ ] #8324
• [ ] Finish EpochManager refactoring (#6910)
• [ ] Have a proper debug UI project (#8356 )
• [ ] #8191

[mzhangmzz]

Adding other ideas here:

• finish the EpochManager refactor https://github.com/near/nearcore/issues/6910