net.h: Add in unknown_resource to coap_context_t

Needed to handle
"RFC7252: 5.8.3.  PUT
 If no resource exists, then the server MAY create a new resource with that
 URI, resulting in a 2.01 (Created) Response Code."

Needed for support of PUT requests to an unknown resource.

Remove structure coap_mid_cache_t as it is not used and refers to the now
obsolete coap_key_t.

include/coap/net.h

@@ -88,16 +88,13 @@ typedef void (*coap_pong_handler_t)(struct coap_context_t *,
 					coap_pdu_t *received,
 					const coap_tid_t id);
 
-#define COAP_MID_CACHE_SIZE 3
-typedef struct {
-  unsigned char flags[COAP_MID_CACHE_SIZE];
-  coap_key_t item[COAP_MID_CACHE_SIZE];
-} coap_mid_cache_t;
-
 /** The CoAP stack's global state is stored in a coap_context_t object */
 typedef struct coap_context_t {
   coap_opt_filter_t known_options;
-  struct coap_resource_t *resources; /**< hash table or list of known resources */
+  struct coap_resource_t *resources; /**< hash table or list of known
+                                          resources */
+  struct coap_resource_t *unknown_resource; /**< can be used for handling
+                                                 unknown resources */
 
 #ifndef WITHOUT_ASYNC
   /**