Delay parallel block download until chain has sufficient work

nMinimumChainWork is an anti-DoS threshold; wait until we have a proposed
tip with more work than that before downloading blocks towards that tip.

src/net_processing.cpp

@@ -480,7 +480,7 @@ void FindNextBlocksToDownload(NodeId nodeid, unsigned int count, std::vector<con
     // Make sure pindexBestKnownBlock is up to date, we'll need it.
     ProcessBlockAvailability(nodeid);
 
-    if (state->pindexBestKnownBlock == NULL || state->pindexBestKnownBlock->nChainWork < chainActive.Tip()->nChainWork) {
+    if (state->pindexBestKnownBlock == NULL || state->pindexBestKnownBlock->nChainWork < chainActive.Tip()->nChainWork || state->pindexBestKnownBlock->nChainWork < UintToArith256(consensusParams.nMinimumChainWork)) {
         // This peer has nothing interesting.
         return;
     }