Move calculated constants from the top of src/headerssync.cpp into src/kernel/chainparams.cpp.
Instead of being hardcoded to mainnet parameters, HeadersSyncState can now vary depending on chain or test. (This means we can reset TARGET_BLOCKS back to the nice round number of 15'000).
Signet and testnets got new HeadersSyncParams constants through temporarily altering headerssync-params.py with corresponding GENESIS_TIME and MINCHAINWORK_HEADERS (based off defaultAssumeValid block height comments, corresponding to nMinimumChainWork). Regtest doesn't have a default assume valid block height, so the values are copied from Testnet 4. Since the constants only affect memory usage, and have very low impact unless dealing with a largely malicious chain, it's not that critical to keep updating them for non-mainnet chains.
GENESIS_TIMEs (UTC):
Testnet3: 1296688602 = datetime(2011, 2, 2)
Testnet4: 1714777860 = datetime(2024, 5, 3)
Signet: 1598918400 = datetime(2020, 9, 1)
These remaining miscellaneous changes were identified by commenting out
the `operator const uint256&` conversion and the `Compare(const uint256&)`
method from `transaction_identifier.h`.
Avoid permanently storing headers from a peer, unless the headers are part of a
chain with sufficiently high work. This prevents memory attacks using low-work
headers.
Designed and co-authored with Pieter Wuille.
