Use vTxHashes to optimize InitData significantly

src/blockencodings.cpp

@@ -99,11 +99,13 @@ ReadStatus PartiallyDownloadedBlock::InitData(const CBlockHeaderAndShortTxIDs& c
 
     std::vector<bool> have_txn(txn_available.size());
     LOCK(pool->cs);
-    for (CTxMemPool::txiter it = pool->mapTx.begin(); it != pool->mapTx.end(); it++) {
-        std::unordered_map<uint64_t, uint16_t>::iterator idit = shorttxids.find(cmpctblock.GetShortID(it->GetTx().GetHash()));
+    const std::vector<std::pair<uint256, CTxMemPool::txiter> >& vTxHashes = pool->vTxHashes;
+    for (size_t i = 0; i < vTxHashes.size(); i++) {
+        uint64_t shortid = cmpctblock.GetShortID(vTxHashes[i].first);
+        std::unordered_map<uint64_t, uint16_t>::iterator idit = shorttxids.find(shortid);
         if (idit != shorttxids.end()) {
             if (!have_txn[idit->second]) {
-                txn_available[idit->second] = it->GetSharedTx();
+                txn_available[idit->second] = vTxHashes[i].second->GetSharedTx();
                 have_txn[idit->second]  = true;
                 mempool_count++;
             } else {