The retaransmission logic was not correct in the lwm2m_engine, and could
lead to faulty behavior in case multiple messages were pending for
retransmission in the queue.
1. Since there is a singe delayed work item for entire retransmission
queue, `coap_pending_next_to_expire` should be called before
scheduling next timeout, to identify which message is going to expire
next (and when). Currently, the engine always set next timeout, based
on timeout from the message being currently re-transmitted.
2. In case the message was re-transmitted several times, and is removed
from the retansmission queue due to a timeout, next retransmission
should be scheduled, in case there are other messages on the queue.
3. Verify the timeout of the earliest message to expire in the
retransmission handler. In case messages from the beginning of the
queue were removed, we might need to schedule the retransmission
again, instead of sending message rightaway.
4. `lwm2m_send_message` is not handling retransmissions anyway, so
there's no need to check send attempts. Instead, verify
retransmission work item is already pending, and update its timeout
if needed.
Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>
afb73d1d09