mavlink: add class for command retransmission

This adds a class to allow for retransmission of outgoing commands.
The sent commands are kept in a timestamped list to check if they are
acked as required by the mavlink protocol.
If they are not acked within a timeout, they can be retransmitted.

src/modules/mavlink/CMakeLists.txt

@@ -1,6 +1,6 @@
 ############################################################################
 #
-#   Copyright (c) 2015 PX4 Development Team. All rights reserved.
+#   Copyright (c) 2015-2017 PX4 Development Team. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions
@@ -53,6 +53,7 @@ px4_add_module(
 		mavlink_log_handler.cpp
 		mavlink_shell.cpp
 		mavlink_ulog.cpp
+		mavlink_command_sender.cpp
 	DEPENDS
 		platforms__common
 	)