add time_stamp api

Author: yph025869

include/cppredis/unit.hpp

@@ -121,6 +121,15 @@ namespace cpp_redis {
 			return value;
 		}
 
+		static size_t get_time_stamp()
+		{
+			std::chrono::time_point<std::chrono::system_clock, std::chrono::seconds> tp = 
+				std::chrono::time_point_cast<std::chrono::seconds>(std::chrono::system_clock::now());
+			
+			auto tmp = std::chrono::duration_cast<std::chrono::seconds>(tp.time_since_epoch());
+			return tmp.count();
+		}
+
 		//正数编负数
 		constexpr int turn(int a)
 		{