MXS-1392 Manage DCB lifetime using refcounts

Now it is also possible to ensure that a DCB stays alive while
a task referring to it is posted from one worker to another.
That will be implemented in a subsequent commit.

include/maxscale/poll_core.hh

@@ -23,12 +23,14 @@ struct MxsPollData : MXS_POLL_DATA
     MxsPollData()
     {
         handler = NULL;
+        free = NULL;
         thread.id = 0;
     }
 
     MxsPollData(mxs_poll_handler_t h)
     {
         handler = h;
+        free = NULL;
         thread.id = 0;
     }
 };