Replace trickle nodes with per-node/message Poisson delays

We used to have a trickle node, a node which was chosen in each iteration of
the send loop that was privileged and allowed to send out queued up non-time
critical messages. Since the removal of the fixed sleeps in the network code,
this resulted in fast and attackable treatment of such broadcasts.

This pull request changes the 3 remaining trickle use cases by random delays:
* Local address broadcast (while also removing the the wiping of the seen filter)
* Address relay
* Inv relay (for transactions; blocks are always relayed immediately)

The code is based on older commits by Patrick Strateman.

src/net.cpp

@@ -36,6 +36,8 @@
 #include <boost/filesystem.hpp>
 #include <boost/thread.hpp>
 
+#include <math.h>
+
 // Dump addresses to peers.dat every 15 minutes (900s)
 #define DUMP_ADDRESSES_INTERVAL 900
 
@@ -1733,11 +1735,6 @@ void ThreadMessageHandler()
             }
         }
 
-        // Poll the connected nodes for messages
-        CNode* pnodeTrickle = NULL;
-        if (!vNodesCopy.empty())
-            pnodeTrickle = vNodesCopy[GetRand(vNodesCopy.size())];
-
         bool fSleep = true;
 
         BOOST_FOREACH(CNode* pnode, vNodesCopy)
@@ -1768,7 +1765,7 @@ void ThreadMessageHandler()
             {
                 TRY_LOCK(pnode->cs_vSend, lockSend);
                 if (lockSend)
-                    g_signals.SendMessages(pnode, pnode == pnodeTrickle || pnode->fWhitelisted);
+                    g_signals.SendMessages(pnode);
             }
             boost::this_thread::interruption_point();
         }
@@ -2384,6 +2381,9 @@ CNode::CNode(SOCKET hSocketIn, const CAddress& addrIn, const std::string& addrNa
     nStartingHeight = -1;
     filterInventoryKnown.reset();
     fGetAddr = false;
+    nNextLocalAddrSend = 0;
+    nNextAddrSend = 0;
+    nNextInvSend = 0;
     fRelayTxes = false;
     pfilter = new CBloomFilter();
     nPingNonceSent = 0;
@@ -2634,3 +2634,7 @@ void DumpBanlist()
     LogPrint("net", "Flushed %d banned node ips/subnets to banlist.dat  %dms\n",
              banmap.size(), GetTimeMillis() - nStart);
 }
+
+int64_t PoissonNextSend(int64_t nNow, int average_interval_seconds) {
+    return nNow + (int64_t)(log1p(GetRand(1ULL << 48) * -0.0000000000000035527136788 /* -1/2^48 */) * average_interval_seconds * -1000000.0 + 0.5);
+}