Add dcb_close_in_owning_thread(DCB*);

Allows a DCB to be closed by a thread other than the owning
thread.

include/maxscale/dcb.h

@@ -222,6 +222,16 @@ int dcb_read(DCB *, GWBUF **, int);
 int dcb_drain_writeq(DCB *);
 void dcb_close(DCB *);
 
+/**
+ * @brief Close DCB in the thread that owns it.
+ *
+ * @param dcb The dcb to be closed.
+ *
+ * @note Even if the calling thread owns the dcb, the closing will
+ *       still be made via the event loop.
+ */
+void dcb_close_in_owning_thread(DCB *dcb);
+
 /**
  * Add a DCB to the owner's list
  *