MXS-1378 Provide access to the current DCB in 2.1

Conceptually this is a cherry-pick of commit
67efd1daeabbc398b8a8fbc0cd02c2af26e4cb6c (2.2), but too much has
changed to actually be able to cherry-pick that commit.

include/maxscale/dcb.h

@@ -351,6 +351,14 @@ bool dcb_foreach(bool (*func)(DCB *, void *), 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
  */