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.cpp

@@ -453,7 +453,7 @@ CNode* FindNode(const CService& addr)
     return NULL;
 }
 
-CNode* ConnectNode(CAddress addrConnect, const char *pszDest, int64 nTimeout)
+CNode* ConnectNode(CAddress addrConnect, const char *pszDest)
 {
     if (pszDest == NULL) {
         if (IsLocal(addrConnect))
@@ -463,10 +463,7 @@ CNode* ConnectNode(CAddress addrConnect, const char *pszDest, int64 nTimeout)
         CNode* pnode = FindNode((CService)addrConnect);
         if (pnode)
         {
-            if (nTimeout != 0)
-                pnode->AddRef(nTimeout);
-            else
-                pnode->AddRef();
+            pnode->AddRef();
             return pnode;
         }
     }
@@ -498,10 +495,7 @@ CNode* ConnectNode(CAddress addrConnect, const char *pszDest, int64 nTimeout)
 
         // Add node
         CNode* pnode = new CNode(hSocket, addrConnect, pszDest ? pszDest : "", false);
-        if (nTimeout != 0)
-            pnode->AddRef(nTimeout);
-        else
-            pnode->AddRef();
+        pnode->AddRef();
 
         {
             LOCK(cs_vNodes);
@@ -615,7 +609,6 @@ void CNode::copyStats(CNodeStats &stats)
     X(nVersion);
     X(strSubVer);
     X(fInbound);
-    X(nReleaseTime);
     X(nStartingHeight);
     X(nMisbehavior);
 }
@@ -773,7 +766,6 @@ void ThreadSocketHandler()
                     pnode->Cleanup();
 
                     // hold in disconnected pool until all refs are released
-                    pnode->nReleaseTime = max(pnode->nReleaseTime, GetTime() + 15 * 60);
                     if (pnode->fNetworkNode || pnode->fInbound)
                         pnode->Release();
                     vNodesDisconnected.push_back(pnode);