MXS-1376 Really close a DCB only at the end of event loop

When dcb_close() is called, the DCB is only marked for closing
and the actual closing takes place only after all event handlers
have been called. That way, the state of the DCB will not change
during event processing but only after.

From a handler perspective this should now be just like it was
when the zombie queue was present.

TODO: There are far too many state variables or variables akin to
      state variables - dcb_role, state, persistentstart, n_close -
      in DCB. A cleanup is warranted.

include/maxscale/dcb.h

@@ -180,6 +180,7 @@ typedef struct dcb
         struct dcb *next; /**< Next DCB in owning thread's list */
         struct dcb *tail; /**< Last DCB in owning thread's list */
     } thread;
+    uint32_t        n_close;         /** How many times dcb_close has been called. */
     skygw_chk_t     dcb_chk_tail;
 } DCB;