Allow the number of incoming connections to be limited

It was already possible to limit outgoing connections. One might want
to do this on home network connections with high bandwidth but low
usage caps.

src/daemon/rpc_command_executor.h

@@ -122,7 +122,9 @@ public:
   bool set_limit(int64_t limit_down, int64_t limit_up);
 
   bool out_peers(uint64_t limit);
-  
+
+  bool in_peers(uint64_t limit);
+
   bool start_save_graph();
   
   bool stop_save_graph();