Drop release times for CNode

It seems there were two mechanisms for assessing whether a CNode
was still in use: a refcount and a release timestamp. The latter
seems to have been there for a long time, as a safety mechanism.

However, this timer also keeps CNode objects alive for far longer
than necessary after disconnects, potentially opening up a DoS
window.

This commit removes the timestamp-based mechanism, and replaces
it with an assert(nRefCount >= 0), to verify that the refcounting
is indeed correctly working.

src/net.h

@@ -37,7 +37,7 @@ bool GetMyExternalIP(CNetAddr& ipRet);
 void AddressCurrentlyConnected(const CService& addr);
 CNode* FindNode(const CNetAddr& ip);
 CNode* FindNode(const CService& ip);
-CNode* ConnectNode(CAddress addrConnect, const char *strDest = NULL, int64 nTimeout=0);
+CNode* ConnectNode(CAddress addrConnect, const char *strDest = NULL);
 void MapPort(bool fUseUPnP);
 unsigned short GetListenPort();
 bool BindListenPort(const CService &bindAddr, std::string& strError=REF(std::string()));
@@ -99,7 +99,6 @@ public:
     int nVersion;
     std::string strSubVer;
     bool fInbound;
-    int64 nReleaseTime;
     int nStartingHeight;
     int nMisbehavior;
 };
@@ -187,8 +186,8 @@ public:
     CSemaphoreGrant grantOutbound;
     CCriticalSection cs_filter;
     CBloomFilter* pfilter;
-protected:
     int nRefCount;
+protected:
 
     // Denial-of-service detection/prevention
     // Key is IP address, value is banned-until-time
@@ -197,7 +196,6 @@ protected:
     int nMisbehavior;
 
 public:
-    int64 nReleaseTime;
     uint256 hashContinue;
     CBlockIndex* pindexLastGetBlocksBegin;
     uint256 hashLastGetBlocksEnd;
@@ -235,7 +233,6 @@ public:
         fSuccessfullyConnected = false;
         fDisconnect = false;
         nRefCount = 0;
-        nReleaseTime = 0;
         nSendSize = 0;
         nSendOffset = 0;
         hashContinue = 0;
@@ -272,7 +269,8 @@ public:
 
     int GetRefCount()
     {
-        return std::max(nRefCount, 0) + (GetTime() < nReleaseTime ? 1 : 0);
+        assert(nRefCount >= 0);
+        return nRefCount;
     }
 
     // requires LOCK(cs_vRecvMsg)
@@ -295,12 +293,9 @@ public:
             msg.SetVersion(nVersionIn);
     }
 
-    CNode* AddRef(int64 nTimeout=0)
+    CNode* AddRef()
     {
-        if (nTimeout != 0)
-            nReleaseTime = std::max(nReleaseTime, GetTime() + nTimeout);
-        else
-            nRefCount++;
+        nRefCount++;
         return this;
     }