Added ping time measurement.

New RPC "ping" command to request ping.
Implemented "pong" message handler.
New "pingtime" field in getpeerinfo, to provide results to user.
New "pingwait" field, to show pings still in flight, to better see newly lagging peers.

src/net.cpp

@@ -620,6 +620,21 @@ void CNode::copyStats(CNodeStats &stats)
     X(nSendBytes);
     X(nRecvBytes);
     stats.fSyncNode = (this == pnodeSync);
+    
+    // It is common for nodes with good ping times to suddenly become lagged,
+    // due to a new block arriving or other large transfer.
+    // Merely reporting pingtime might fool the caller into thinking the node was still responsive,
+    // since pingtime does not update until the ping is complete, which might take a while.
+    // So, if a ping is taking an unusually long time in flight,
+    // the caller can immediately detect that this is happening.
+    int64 nPingUsecWait = 0;
+    if ((0 != nPingNonceSent) && (0 != nPingUsecStart)) {
+        nPingUsecWait = GetTimeMicros() - nPingUsecStart;
+    }
+    
+    // Raw ping time is in microseconds, but show it to user as whole seconds (Bitcoin users should be well used to small numbers with many decimal places by now :)
+    stats.dPingTime = (((double)nPingUsecTime) / 1e6);
+    stats.dPingWait = (((double)nPingUsecWait) / 1e6);
 }
 #undef X