net: coap: Add initial tx time to coap_pending structure

So far, coap_pending structure kept track only of the timeout interval
between two consecutive retransmissions. Calculations inside
`coap_pending_next_to_expire` relied only on this value. This approach
gives incorrect results though, in case multiple messages are pending
for retransmission.

For instance, assuming initial retransmission timeout is set to 2
seconds. If some message had been retransmitted already, its timeout
would be increased to 4 seconds. Any new message added to the pending
list would have a retransmission timeout set to 2 seconds, and will be
returned as a first message to expire, no matter how long the initial
message was already on the list.

To resolve this, add a `t0` field to the coap_pending structure. This
field is initialized to the initial transmission time, and is increased
on each retransmission by the retransmission timeout.
`coap_pending_next_to_expire` uses this value to calculate absolute
time, when the next retransmission should take place, and based on this
information returns correctly first pending message to expire.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>

include/net/coap.h

@@ -233,7 +233,8 @@ typedef int (*coap_reply_t)(const struct coap_packet *response,
  */
 struct coap_pending {
 	struct sockaddr addr;
-	s32_t timeout;
+	u32_t t0;
+	u32_t timeout;
 	u16_t id;
 	u8_t *data;
 	u16_t len;