MQTT: fix MQTT_PUBLISHED_EVENT not always being posted

- Some PUBLISHED events would not be posted due to outbox messages being deleted before receiving PUBCOMP.
 - Current pending_message is no longer used to check for acknowledgements, as it doesn't work with multiple or out of order messages.

Closes https://github.com/espressif/esp-mqtt/issues/132

lib/include/mqtt_outbox.h

@@ -30,6 +30,7 @@ typedef struct outbox_message {
 typedef enum pending_state {
     QUEUED,
     TRANSMITTED,
+    ACKNOWLEDGED,
     CONFIRMED
 } pending_state_t;