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.
  • [testing] Improve interface of Chain test_utils #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 (Rigorously define "canonical chain" and ensure correct maintenance and use of canonical chain #8154). Have a isolated component just to manage topology.
• Finish ShardsManager refactoring to move it onto separate actix (Move ShardsManager chunk management logic to a separate Actix thread #7662)
• Write a localized integration test for ShardsManager without creating Client #8324
• Finish EpochManager refactoring (Figure out what to do with EpochManager medium-term #6910)
• Have a proper debug UI project (Tracking issue for new debug UI (tools/debug-ui) #8356 )
• Improve header sync code modularity #8191

[mzhangmzz]

Adding other ideas here:

• finish the EpochManager refactor Figure out what to do with EpochManager medium-term #6910