Fork infrastructure

[arvidn]

These commits are best reviewed one at a time.

These two commits:

1. Removes the soft-fork logic for the 1.7-soft fork (which has activated on mainnet)
2. Adds the constants for the next soft fork (soft_fork3) as well as the 2.0 hard fork and plot filter adjustment heights.

This patch does not implement any of the soft-forks or hard forks behavior, they will be separate PRs. But since multiple people coordinate on these constants, it makes sense to land them separately, first.

[coveralls-official]

Pull Request Test Coverage Report for Build 5017824603

• 21 of 21 (100.0%) changed or added relevant lines in 9 files are covered.
• 12 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.008%) to 85.002%

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Files with Coverage Reduction	New Missed Lines	%
tests/core/mempool/test_mempool.py	1	95.93%
chia/timelord/timelord.py	2	73.02%
chia/rpc/rpc_server.py	3	87.46%
chia/full_node/full_node.py	6	85.36%

Totals
Change from base Build 5016993464:	0.008%
Covered Lines:	75757
Relevant Lines:	88925

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💛 - Coveralls