net: samples: fix coap_client early unref packet issue

1. After calling sendto(), the packet will be unreferenced.
   Call coap_pending_cycle() to make sure the packet is referenced until
   we got the response from the peer or retransmission timeout.
2. In retransmit_request(), we rearrange the order to avoid the issue.
3. Remove IPv4 configuration for source code is expecting IPv6
4. Correct a typo in README.rst
5. Initialize the interface w/ IPv6 address

Signed-off-by: Robert Chou <robert.ch.chou@acer.com>