Only send live checks when the connection has been silent for a while.

Also:
* Made the timeout configurable
* Removed the stale randomcharacters config option
* Switched to time_t for time values, even though it doesn't really
matter that much
* Halved the keepAliveTimer frequency

src/PlayerManager.hpp

@@ -14,7 +14,7 @@ struct PlayerView {
     std::list<CNSocket*> viewable;
     std::list<int32_t> viewableNPCs;
     Player *plr;
-    uint64_t lastHeartbeat;
+    time_t lastHeartbeat;
 };