net: coap: Coap pending allocation fix

When sending 2 or more confirmable message before first one is
writed to socket all messages use same coap pending structure.
Now coap_pending_init() set data pointer which lock allocation
by each call.

Using data pointer for detecting free is more stable than timeout.
Timeout is initialized only before first socket send. Queued packet
may be triggered later than other and may cause that same block is
allocated multiple time.

Signed-off-by: Juha Heiskanen <juha.heiskanen@nordicsemi.no>