[CHIA-3793] after the hard fork, disallow block references

[arvidn]

Purpose:

With hard fork 2, we want to disallow block references. We have not seen any benefit or use of them so far. We currently compress block generators using (internal) back references. Deduplicating structures.

Disallowing block generators has the following upsides:

• It's much simpler to validate a block, without needing access to the whole chain
• A node don't necessarily need to keep every block around to validate and be a part of the chain
• It's cheaper to validate blocks, without requiring additional DB lookups for previous blocks

There are two additional commits in this PR.

1. Disable simulations of har fork 2 activation, since the existing chains do contain block references, we can't use them in tests after the hard fork. These will be re-enabled once we have new test block chains specifically made with the new consensus rules
2. fix a flaky (and awkward) test related to rate limits

[github-actions]

File	Coverage	Missing Lines
chia/consensus/block_body_validation.py	66.7%	lines 350
chia/consensus/blockchain.py	83.3%	lines 710
chia/simulator/block_tools.py	33.3%	lines 389-390

Total	Missing	Coverage
18 lines	4 lines	77%