GatewayConnection: mark received message as const
GatewayConnection: mark received message as const

Signed-off-by: Jan Viktorin <3eab6ac54c005c4408f608556a6663b7f2abede7@fit.vutbr.cz>
Reviewed-by: Richard Wolfert <295b11fe6efda7c962c8f602eeeb610998fc60f5@stud.fit.vutbr.cz>
Reviewed-by: Jozef Halaj <d60338334aa79f5c61824693142b417ad5db682f@stud.fit.vutbr.cz>

BeeeOn/server,BeeeOn/server,BeeeOn/server,BeeeOn/server