rpc: add a command to get info about the current blockchain

About the tip of the main chain, and the last N blocks

src/daemon/command_parser_executor.h

@@ -121,6 +121,8 @@ public:
   bool print_coinbase_tx_sum(const std::vector<std::string>& args);
 
   bool alt_chain_info(const std::vector<std::string>& args);
+
+  bool print_blockchain_dynamic_stats(const std::vector<std::string>& args);
 };
 
 } // namespace daemonize