Since model struct most of member should not change at run time,
so mark as const will be suitable and safely.
Signed-off-by: Lingao Meng <menglingao@xiaomi.com>
Convert model_ackd_send and model_send from health_cli.c to a common API
to get rid of code duplication in other client models that implement
synchronous messages' sending.
Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>