Ping automatically every 2 minutes (unconditionally)

... instead of after 30 minutes of no sending, for latency measurement
and keep-alive. Also, disconnect if no reply arrives within 20 minutes,
instead of 90 of inactivity (for peers supporting the 'pong' message).

src/net.h

@@ -28,6 +28,7 @@
 #include <boost/signals2/signal.hpp>
 #include <openssl/rand.h>
 
+
 class CAddrMan;
 class CBlockIndex;
 class CNode;
@@ -36,6 +37,10 @@ namespace boost {
     class thread_group;
 }
 
+/** Time between pings automatically sent out for latency probing and keepalive (in seconds). */
+static const int PING_INTERVAL = 2 * 60;
+/** Time after which to disconnect, after waiting for a ping response (or inactivity). */
+static const int TIMEOUT_INTERVAL = 20 * 60;
 /** The maximum number of entries in an 'inv' protocol message */
 static const unsigned int MAX_INV_SZ = 50000;
 /** -upnp default */
@@ -217,7 +222,6 @@ public:
 
     int64_t nLastSend;
     int64_t nLastRecv;
-    int64_t nLastSendEmpty;
     int64_t nTimeConnected;
     CAddress addr;
     std::string addrName;
@@ -273,10 +277,14 @@ public:
     CCriticalSection cs_inventory;
     std::multimap<int64_t, CInv> mapAskFor;
 
-    // Ping time measurement
+    // Ping time measurement:
+    // The pong reply we're expecting, or 0 if no pong expected.
     uint64_t nPingNonceSent;
+    // Time (in usec) the last ping was sent, or 0 if no ping was ever sent.
     int64_t nPingUsecStart;
+    // Last measured round-trip time.
     int64_t nPingUsecTime;
+    // Whether a ping is requested.
     bool fPingQueued;
 
     CNode(SOCKET hSocketIn, CAddress addrIn, std::string addrNameIn = "", bool fInboundIn=false) : ssSend(SER_NETWORK, INIT_PROTO_VERSION), setAddrKnown(5000)
@@ -288,7 +296,6 @@ public:
         nLastRecv = 0;
         nSendBytes = 0;
         nRecvBytes = 0;
-        nLastSendEmpty = GetTime();
         nTimeConnected = GetTime();
         addr = addrIn;
         addrName = addrNameIn == "" ? addr.ToStringIPPort() : addrNameIn;