[experiment] Produce a block regardless of how many transactions are in it

[patrick-ogrady]

If we continuously produce blocks, we will:

• prevent large AWM diff lookbacks (last block may reference an old p-chain height)
• never fallback to Snowman consensus
• prevent prolonged post-bootstrap readiness wait if no txs (need to see 60 seconds of blocks)
• easier to implement chain/validator-driven txs

We can pretty easily implement this now that we have millisecond block times.

[patrick-ogrady]

This will allow us to remove all our logic during E2E to issue txs to make nodes ready as well.

[patrick-ogrady]

We should also require that empty blocks be a min duration after the previous block (likely much larger than the min diff time to avoid spam).