net: lib: coap: CoAP client, multiple request handling

Use only single thread for handling polling of the sockets.
Each client will have only 1 active socket which to poll.
Each client can have multiple simultaneous requests ongoing.
The client only has one buffer for receiving and one buffer for sending.
Therefore the messages are reformed when resending.

Signed-off-by: Jarno Lämsä <jarno.lamsa@nordicsemi.no>