spec: define syncing protocols

[lasarojc]

Was tendermint/tendermint#8219

Summary

Our current spec has no mention for the syncing protocols that CometBFT uses: block sync and state sync. It would be good to write one up these so people can use it to quickly understand how each of them work.

• Block sync specification (@jmalicevic)
• State sync specification (@cmwaters)

[josef-widder]

For Informal's contract with the ICF a long time ago, we have written a spec for FastSync, which is now called "Block sync". I think this might be a good starting point for a high-level specification for Block sync. I am happy to discuss the old work whenever someone takes on this issue.

[josef-widder]

For blocksync there is also a description of the current behavior by @jmalicevic. If we could compile this, and the part on temporal properties (invariants) from the Fastsync spec above, we should be able to get a blocksync spec quite quickly.

[cason]

Regarding state sync, as part of #28, I've found the following documentation and specs:

• docs/core/state-sync.md:

  • very basic information of how to enable state sync
  • lacks from a good introduction and general description
  • check if in line with the [statesync] block on the configuration

• spec/p2p/messages/state-sync.md:

  • documents the messages exchanged by the protocol, not really its operation
  • referred as existing specification on the remaining documents

• spec/abci/abci++_basic_concepts.md:

  • State-sync methods section, a good summary of state sync
  • reference for the ABCI methods used by block sync
    • the specification of these methods add some relevant details

• spec/abci/abci++_app_requirements.md:

  • State Sync section is probably the best description and documentation we have
  • the point of view, however, is the application, target for application developers