Eliminate TX trickle bypass, sort TX invs for privacy and priority.

Previously Bitcoin would send 1/4 of transactions out to all peers
 instantly.  This causes high overhead because it makes >80% of
 INVs size 1.  Doing so harms privacy, because it limits the
 amount of source obscurity a transaction can receive.

These randomized broadcasts also disobeyed transaction dependencies
 and required use of the orphan pool.  Because the orphan pool is
 so small this leads to poor propagation for dependent transactions.

When the bypass wasn't in effect, transactions were sent in the
 order they were received.  This avoided creating orphans but
 undermines privacy fairly significantly.

This commit:
 Eliminates the bypass. The bypass is replaced by halving the
  average delay for outbound peers.

 Sorts candidate transactions for INV by their topological
  depth then by their feerate (then hash); removing the
  information leakage and providing priority service to
  higher fee transactions.

 Limits the amount of transactions sent in a single INV to
  7tx/sec (and twice that for outbound); this limits the
  harm of low fee transaction floods, gives faster relay
  service to higher fee transactions. The 7 sounds lower
  than it really is because received advertisements need
  not be sent, and because the aggregate rate is multipled
  by the number of peers.

src/main.cpp

@@ -5560,6 +5560,29 @@ bool ProcessMessages(CNode* pfrom)
     return fOk;
 }
 
+class CompareInvMempoolOrder
+{
+    CTxMemPool *mp;
+public:
+    CompareInvMempoolOrder(CTxMemPool *mempool)
+    {
+        mp = mempool;
+    }
+
+    bool operator()(const CInv &a, const CInv &b)
+    {
+        if (a.type != MSG_TX && b.type != MSG_TX) {
+            return false;
+        } else {
+            if (a.type != MSG_TX) {
+                return true;
+            } else if (b.type != MSG_TX) {
+                return false;
+            }
+            return mp->CompareDepthAndScore(a.hash, b.hash);
+        }
+    }
+};
 
 bool SendMessages(CNode* pto)
 {
@@ -5790,42 +5813,31 @@ bool SendMessages(CNode* pto)
             bool fSendTrickle = pto->fWhitelisted;
             if (pto->nNextInvSend < nNow) {
                 fSendTrickle = true;
-                pto->nNextInvSend = PoissonNextSend(nNow, AVG_INVENTORY_BROADCAST_INTERVAL);
+                // Use half the delay for outbound peers, as their is less privacy concern for them.
+                pto->nNextInvSend = PoissonNextSend(nNow, INVENTORY_BROADCAST_INTERVAL >> !pto->fInbound);
             }
             LOCK(pto->cs_inventory);
-            vInv.reserve(std::min<size_t>(1000, pto->vInventoryToSend.size()));
+            if (fSendTrickle && pto->vInventoryToSend.size() > 1) {
+                // Topologically and fee-rate sort the inventory we send for privacy and priority reasons.
+                CompareInvMempoolOrder compareInvMempoolOrder(&mempool);
+                std::stable_sort(pto->vInventoryToSend.begin(), pto->vInventoryToSend.end(), compareInvMempoolOrder);
+            }
+            vInv.reserve(std::min<size_t>(INVENTORY_BROADCAST_MAX, pto->vInventoryToSend.size()));
             vInvWait.reserve(pto->vInventoryToSend.size());
             BOOST_FOREACH(const CInv& inv, pto->vInventoryToSend)
             {
                 if (inv.type == MSG_TX && pto->filterInventoryKnown.contains(inv.hash))
                     continue;
-
-                // trickle out tx inv to protect privacy
-                if (inv.type == MSG_TX && !fSendTrickle)
-                {
-                    // 1/4 of tx invs blast to all immediately
-                    static uint256 hashSalt;
-                    if (hashSalt.IsNull())
-                        hashSalt = GetRandHash();
-                    uint256 hashRand = ArithToUint256(UintToArith256(inv.hash) ^ UintToArith256(hashSalt));
-                    hashRand = Hash(BEGIN(hashRand), END(hashRand));
-                    bool fTrickleWait = ((UintToArith256(hashRand) & 3) != 0);
-
-                    if (fTrickleWait)
-                    {
-                        vInvWait.push_back(inv);
-                        continue;
-                    }
+                // No reason to drain out at many times the network's capacity,
+                // especially since we have many peers and some will draw much shorter delays.
+                if (vInv.size() >= INVENTORY_BROADCAST_MAX || (inv.type == MSG_TX && !fSendTrickle)) {
+                    vInvWait.push_back(inv);
+                    continue;
                 }
 
                 pto->filterInventoryKnown.insert(inv.hash);
 
                 vInv.push_back(inv);
-                if (vInv.size() >= 1000)
-                {
-                    pto->PushMessage(NetMsgType::INV, vInv);
-                    vInv.clear();
-                }
             }
             pto->vInventoryToSend = vInvWait;
         }