Introduce a more generic mutex for protecting coap_client structure.
This allows to avoid a certain race condition when sending consecutive
CoAP requests. The case was, that when a CoAP receive thread notified
the application that a complete response was received, and the
application wanted to send another request from the application thread,
the consecutive call to coap_client_req() might've failed if the
application thread has higher priority than the CoAP receive thread
because of the request context cleanup is done after calling the
application callback.
Having a mutex, which is locked while processing the response, and when
attempting to send a new request allows to synchronize threads as
expected.
As this new mutex seemed redundant with the more specialized send_mutex
already present in the coap_client, the latter was removed (i. e.
transmission is also protected with the new mutex).
Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>
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