ABCI v3: Enabling new use cases for application developers

[thanethomson]

High-level tracking issue for multi-year efforts toward enabling new use cases for application developers.

The ABCI model has been quite successful thus far, but suffers from some limitations that we believe could be overcome by ABCI 2.0 (a.k.a. ABCI++).

• ABCI 1.0 with Prepare/ProcessProposal in CometBFT v0.37 (see v0.37 release plan #41)
  • ABCI++ Prepare/Process Proposal tendermint/tendermint#9053
• ABCI 2.0 built on top of 1.0, with FinalizeBlock and Vote Extensions, in CometBFT v0.38 (see v0.38 release plan #33)
  • ABCI++ vote extensions & FinalizeBlock #12
• Post ABCI 2.0 (somewhat) urgent improvements
  • ApplySnapshotChunk should panic when an error is returned #848
• ABCI 3.0 and other non-urgent improvements
  • abci++: Define and implement pruning strategy for stored extended commits #50
  • Implement same-block execution mode #446
  • ABCI++: application to control how many txs it receives in PrepareProposal #980
  • Decide on critical additional VoteExtension metrics tendermint/tendermint#8498
  • Disconnect from peer if it sends vote extension data when disabled tendermint/tendermint#8565
  • Make vote extensions (already) available via FinalizeBlock
    • Understand use cases (mainly encrypted mempools)
    • If we make them available a FinalizeBlock, should be remove them from PrepareProposal? (redundant?)
  • ABCI: Allow proposers to attach additional metadata to proposals #1670
  • The role of Commit method in CometBFT ABCI #1957
  • Other improvements to ABCI proposed by the Skip team (see doc)
  • New ABCI method to surface to the application that there was a consensus error #2051
  • abci: Giving applications access to the P2P layer #1112
  • [ABCI]: replace int64 with uint64 where possible  #2059
  • Support for applications to detect if node is in sync with network or replaying blocks #1247
  • Decouple ABCI from Consensus: Support for customizing the fields inside `Block.Header` #635
  • Ensure that all vote extensions in the commit info are verified #2523

Original issue: tendermint/tendermint#9886