net: lwm2m: fix message release for separate reply

In the case of a proxy server translating HTTP -> COAP (known in
the code as "separate reply"), we were leaking lwm2m_message structures.
This was due to pending objects being cleared out during the first ACK,
and no other way of finding a matching message when the follow up packet
was received.  Let's add a second match for reply to make sure we can
find our matching message resources.

NOTE: This change renames find_msg_from_pending() to find_msg() and
makes it a static function as it's only used by the lwm2m_engine.

Signed-off-by: Michael Scott <michael.scott@linaro.org>