net: lwm2m: update client tx timestamp before sending message

Update client tx timestamp right after message is added to list
of outgoing messages. Delay between when message is generated and
sent is negligible. This will prevents bugs that appear when using
queue mode, where internal engine logic depends on timestamp being
updated when message is generated.

Signed-off-by: Marin Jurjević <marin.jurjevic@hotmail.com>