scripted-diff: replace chainActive -> ::ChainActive()

Though at the moment ChainActive() simply references `g_chainstate.m_chain`,
doing this change now clears the way for multiple chainstate usage and allows
us to script the diff.

-BEGIN VERIFY SCRIPT-
git grep -l "chainActive" | grep -E '(h|cpp)$' | xargs sed -i '/chainActive =/b; /extern CChain& chainActive/b; s/\(::\)\{0,1\}chainActive/::ChainActive()/g'
-END VERIFY SCRIPT-

src/policy/fees.cpp

@@ -526,7 +526,7 @@ void CBlockPolicyEstimator::processTransaction(const CTxMemPoolEntry& entry, boo
     if (txHeight != nBestSeenHeight) {
         // Ignore side chains and re-orgs; assuming they are random they don't
         // affect the estimate.  We'll potentially double count transactions in 1-block reorgs.
-        // Ignore txs if BlockPolicyEstimator is not in sync with chainActive.Tip().
+        // Ignore txs if BlockPolicyEstimator is not in sync with ::ChainActive().Tip().
         // It will be synced next time a block is processed.
         return;
     }