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.h

@@ -113,7 +113,7 @@ struct CNodeSignals
 {
     boost::signals2::signal<int ()> GetHeight;
     boost::signals2::signal<bool (CNode*), CombinerAll> ProcessMessages;
-    boost::signals2::signal<bool (CNode*, bool), CombinerAll> SendMessages;
+    boost::signals2::signal<bool (CNode*), CombinerAll> SendMessages;
     boost::signals2::signal<void (NodeId, const CNode*)> InitializeNode;
     boost::signals2::signal<void (NodeId)> FinalizeNode;
 };
@@ -391,6 +391,8 @@ public:
     CRollingBloomFilter addrKnown;
     bool fGetAddr;
     std::set<uint256> setKnown;
+    int64_t nNextAddrSend;
+    int64_t nNextLocalAddrSend;
 
     // inventory based relay
     CRollingBloomFilter filterInventoryKnown;
@@ -398,6 +400,7 @@ public:
     CCriticalSection cs_inventory;
     std::set<uint256> setAskFor;
     std::multimap<int64_t, CInv> mapAskFor;
+    int64_t nNextInvSend;
     // Used for headers announcements - unfiltered blocks to relay
     // Also protected by cs_inventory
     std::vector<uint256> vBlockHashesToAnnounce;
@@ -791,4 +794,7 @@ public:
 
 void DumpBanlist();
 
+/** Return a timestamp in the future (in microseconds) for exponentially distributed events. */
+int64_t PoissonNextSend(int64_t nNow, int average_interval_seconds);
+
 #endif // BITCOIN_NET_H