DOS: Respect max per-peer blocks in flight limit

Don't allow immediate inv driven block downloads if
a peer already has MAX_BLOCKS_IN_TRANSIT_PER_PEER
active downloads.  Prevents bogus inv spam from
blowing up block transfer tracking data structures.

src/main.cpp

@@ -3574,7 +3574,9 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
                     // doing this will result in the received block being rejected as an orphan in case it is
                     // not a direct successor.
                     pfrom->PushMessage("getheaders", chainActive.GetLocator(pindexBestHeader), inv.hash);
-                    if (chainActive.Tip()->GetBlockTime() > GetAdjustedTime() - Params().TargetSpacing() * 20) {
+                    CNodeState *nodestate = State(pfrom->GetId());
+                    if (chainActive.Tip()->GetBlockTime() > GetAdjustedTime() - Params().TargetSpacing() * 20 &&
+                        nodestate->nBlocksInFlight < MAX_BLOCKS_IN_TRANSIT_PER_PEER) {
                         vToFetch.push_back(inv);
                         // Mark block as in flight already, even though the actual "getdata" message only goes out
                         // later (within the same cs_main lock, though).