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/settings.hpp

@@ -7,6 +7,7 @@ namespace settings {
     extern int DBSAVEINTERVAL;
     extern int SHARDPORT;
     extern std::string SHARDSERVERIP;
+    extern time_t TIMEOUT;
     extern int PLAYERDISTANCE;
     extern int NPCDISTANCE;
     extern int SPAWN_X;