get_block_generator() fork detection

[arvidn]

Purpose:

fix logic in get_block_generator() that determines whether we're looking up a block from a fork or the main chain.

Current Behavior:

We only consider a block lookup to be in the main chain if its previous block is in the cache.

New Behavior:

We consider block lookups part of the main chain if the previous block is in the database and its hash is the same as the height-to-hash map for that height.

testing

This change was tested by issuing the following RPC command:

chia rpc full_node get_puzzle_and_solution '{"coin_id": "0xdc43c91390490b8bbec628bf3c0c2388578548c1889f0ea40c6cfc8fac409947", "height": 280858}'

in main this freezes with this patch it returns the result quickly.

[coveralls-official]

Pull Request Test Coverage Report for Build 6909885509

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 18 unchanged lines in 8 files lost coverage.
• Overall coverage increased (+0.03%) to 90.359%

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Files with Coverage Reduction	New Missed Lines	%
chia/data_layer/data_layer.py	1	81.68%
chia/timelord/timelord_launcher.py	1	69.77%
chia/wallet/wallet_node.py	1	87.94%
tests/simulation/test_simulation.py	1	96.52%
chia/data_layer/data_layer_wallet.py	2	91.09%
chia/server/node_discovery.py	2	79.04%
chia/full_node/full_node.py	5	84.98%
chia/server/server.py	5	81.18%

Totals
Change from base Build 6898618925:	0.03%
Covered Lines:	93749
Relevant Lines:	103709

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💛 - Coveralls

[wjblanke]

works for me