Fix: allow epochs to transition in a single block, and boot cleanup

[kantai]

This PR does a couple of things related to epoch transitions and boot code in the subnet node:

1. Removes boot contracts that definitely don't apply to a subnet (PoX, lockups, PoX-2, genesis)
2. Sets the default epoch transition heights such that Epoch2.1 starts in the genesis block of a subnet node.
3. Updates the epoch transition logic to allow nodes to immediately transition to Epoch2.1 in the first block (i.e., go straight from Epoch2 to Epoch2.1)

The integration tests all test this behavior now, because they used the default regtest epoch configurations.

[obycode]

This is great. Thanks for this @kantai! This can close #196, and we can break out the remaining discussion on BNS into a separate issue. Before removing the genesis contract, I wonder if marketing has any fun ideas to include in the Hiro subnet.

[obycode]

Thanks for doing this cleanup!!

[obycode]

Hmm... BNS is a bit weird within a subnet. Could we have a BNS contract in here that just implements transfers and the subnet-asset trait, so that BNS names could be traded in a subnet? Wallets should probably use L1 BNS to lookup addresses when interacting on a subnet. I'd like to think about that some more, and either way, it doesn't need to be part of this PR, just noting some thoughts here.

[kantai]

Yeah, that was my thought -- do you think it makes sense to delete the BNS contract in this PR as well for now, and leave the subnet-BNS contract as a separate issue?

[obycode]

Yes, that sounds good. Whatever we end up with, we definitely don't want this BNS contract.

[obycode]

I'll open an issue for it.

[codecov-commenter]

Codecov Report

Merging #229 (ae60247) into master (54c653c) will increase coverage by 1.06%.
The diff coverage is n/a.

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@@            Coverage Diff             @@
##           master     #229      +/-   ##
==========================================
+ Coverage   91.19%   92.26%   +1.06%     
==========================================
  Files           6        6              
  Lines         284      336      +52     
==========================================
+ Hits          259      310      +51     
- Misses         25       26       +1     

Impacted Files	Coverage Δ
core-contracts/contracts/helper/simple-ft.clar	75.00% <ø> (-1.48%)	⬇️
...contracts/contracts/helper/simple-nft-no-mint.clar	66.66% <ø> (ø)
core-contracts/contracts/helper/simple-nft.clar	73.33% <ø> (ø)
core-contracts/contracts/multi-miner.clar	97.05% <ø> (ø)
core-contracts/contracts/subnet.clar	94.96% <ø> (+0.81%)	⬆️

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