net: coap: Fix response matching algorithm

The algorithm for matching request with response was incorrect, which
could lead to false matches (for example if request had a token, and
piggybacked reply had no token but matching message ID only, that would
still be counted as a match).

This commit fixes it. The request/reply matching is implemented based on
RFC now, with separate conditions for piggybacked/separate responses.

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