iot/zoap: Fix requiring that the buffer is unchanged for retransmit

It is possible that the buffer waiting for retransmission is modified
after it is sent, for example, it can be compacted by 6lo, and our
assumption of where is the message ID is located in the buffer is no
longer valid.

As the message ID is the only information that is necessary for
keeping track of retransmissions, we keep a copy of it in the pending
struct, as well as the destination address of the retransmission.

Change-Id: Id33d54353404628673541225a1a05e27ee08765f
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>