headerssync: Make HeadersSyncState more flexible and move constants

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)

src/net_processing.cpp

@@ -2654,7 +2654,7 @@ bool PeerManagerImpl::TryLowWorkHeadersSync(Peer& peer, CNode& pfrom, const CBlo
             // advancing to the first unknown header would be a small effect.
             LOCK(peer.m_headers_sync_mutex);
             peer.m_headers_sync.reset(new HeadersSyncState(peer.m_id, m_chainparams.GetConsensus(),
-                chain_start_header, minimum_chain_work));
+                m_chainparams.HeadersSync(), chain_start_header, minimum_chain_work));
 
             // Now a HeadersSyncState object for tracking this synchronization
             // is created, process the headers using it as normal. Failures are