Introduce TxBlock and RootBlock

[patrick-ogrady]

Closes: #52
Replaces: #136

TODO

• Add issue to remove tx gossip between validators, just rely on API/Client to distribute
• invariant: tx block never needs to interact with root block
• add height to txBlock hash to avoid massive compaction when storing/deleting
• create genesis txBlock to avoid empty parent checks (height == 0)
• resolve txBlocks from disk
• avoid using multiple layers of MerkleDB on TxBlocks (will generate IDs on each layer) -> should use VersionDB
  • @dboehm-avalabs is fixing this and we won't need to change it
• Add metric for verification delay (time between request verification and verification ready to proceed)
• Use streamed blocks to ack txs (or test larger write buffer) or batch acks based on what is available
  • also batch submit
• Add metric for build time of blocks issued to consensus
• Ensure verify delay time exits immediately after checking blocks (don't allow redos)
• Handle closed Chan
• Carry view forward across all TxBlocks (don't allow forks to be processed?)
  • can assert each node gets 1 txBlock at each height
• Verify signatures during Last so we don't block RootBlock.Verify
• Use millisecond timestamps (UnixMilli) + limit 1 block per X ms (avoid surge of seconds)
• recover txs from dropped txBlocks (into mempool)
• only gossip to next validators, non-stake can fetch chunks

[patrick-ogrady]

There should be no tx blocks missing:

[patrick-ogrady]

gzip compression should be disabled?

[patrick-ogrady]

Amount of state changes we can process rapidly degrades:

[patrick-ogrady]

Verify and build time explode:

[github-actions]

This PR has become stale because it has been open for 30 days with no activity. Adding the lifecycle/frozen label will exempt this PR from future lifecycle events..