MXS-1378 Provide access to the current DCB

This will be used by a subsequent `session_get_current()` and
`session_get_current_id()` for obtaining the current SESSION and
session id, respectively. The latter of those will be used by the
logging mechanism for logging the session id in conjunction with
messages.

include/maxscale/dcb.h

@@ -293,6 +293,14 @@ void dcb_foreach_parallel(bool (*func)(DCB *dcb, void *data), void **data);
  */
 int dcb_get_port(const DCB *dcb);
 
+/**
+ * @brief Return the DCB currently being handled by the calling thread.
+ *
+ * @return A DCB, or NULL if the calling thread is not currently handling
+ *         a DCB or if the calling thread is not a polling/worker thread.
+ */
+DCB* dcb_get_current();
+
 /**
  * DCB flags values
  */