Use the cached consensusBranchId in DisconnectBlock

If a node is started with a set of network upgrades that don't match the
serialized chain (such as when we implement NU rollbacks on testnet),
RewindBlockIndex will disconnect each block in the chain until it
reaches the most recent block that agrees with the node's set of network
upgrades. However, the blocks themselves should be disconnected using
the consensus branch ID that they were connected with, which is
persisted alongside the chain and reconstructed in LoadBlockIndex.

src/main.cpp

@@ -2461,7 +2461,9 @@ static DisconnectResult DisconnectBlock(const CBlock& block, CValidationState& s
         view.PopAnchor(SaplingMerkleTree::empty_root(), SAPLING);
     }
 
-    auto consensusBranchId = CurrentEpochBranchId(pindex->nHeight, chainparams.GetConsensus());
+    // This is guaranteed to be filled by LoadBlockIndex.
+    assert(pindex->nCachedBranchId);
+    auto consensusBranchId = pindex->nCachedBranchId.get();
 
     if (chainparams.GetConsensus().NetworkUpgradeActive(pindex->nHeight, Consensus::UPGRADE_HEARTWOOD)) {
         view.PopHistoryNode(consensusBranchId);