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/headerssync.h

@@ -136,7 +136,8 @@ public:
      * minimum_required_work: amount of chain work required to accept the chain
      */
     HeadersSyncState(NodeId id, const Consensus::Params& consensus_params,
-            const CBlockIndex* chain_start, const arith_uint256& minimum_required_work);
+            const HeadersSyncParams& params, const CBlockIndex* chain_start,
+            const arith_uint256& minimum_required_work);
 
     /** Result data structure for ProcessNextHeaders. */
     struct ProcessingResult {
@@ -179,8 +180,8 @@ protected:
     /** The (secret) offset on the heights for which to create commitments.
      *
      * m_header_commitments entries are created at any height h for which
-     * (h % HEADER_COMMITMENT_PERIOD) == m_commit_offset. */
-    const unsigned m_commit_offset;
+     * (h % m_params.commitment_period) == m_commit_offset. */
+    const size_t m_commit_offset;
 
 private:
     /** Clear out all download state that might be in progress (freeing any used
@@ -214,6 +215,9 @@ private:
     /** We use the consensus params in our anti-DoS calculations */
     const Consensus::Params& m_consensus_params;
 
+    /** Parameters that impact memory usage for a given chain, especially when attacked. */
+    const HeadersSyncParams m_params;
+
     /** Store the last block in our block index that the peer's chain builds from */
     const CBlockIndex* m_chain_start{nullptr};