Bluetooth: Mesh: Fix publication period measurement

Period measurement should be started right after a message is updated.
That is because `struct bt_mesh_send_cb.start` callback is called from
the advertiser thread which is not necessary to be scheduled immeditely
and a user may see some delays between update handler calls.

The publication timer still needs to be scheduled after a message being
actually sent out to the air so that access layer doesn't overflow the
advertiser's buffer. To achieve some specific use cases, where a
published message needs to be retransmitted with 50ms interval, a user
has to switch off network retransmissions to make the legacy advertiser
sleep for the shortest possible time.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>