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/dcb.h

@@ -216,7 +216,6 @@ void dcb_global_init();
 int dcb_write(DCB *, GWBUF *);
 DCB *dcb_accept(DCB *listener);
 DCB *dcb_alloc(dcb_role_t, struct servlistener *);
-void dcb_free_all_memory(DCB *dcb);
 DCB *dcb_connect(struct server *, struct session *, const char *);
 int dcb_read(DCB *, GWBUF **, int);
 int dcb_drain_writeq(DCB *);

include/maxscale/poll_core.h

@@ -131,7 +131,8 @@ static inline void poll_inc_ref(MXS_POLL_DATA* data)
  * @param data  The poll data whose reference count should be decreased.
  *
  * @return The previous reference count. If the returned value is 1, then
- *         the caller is the last user of the data.
+ *         the caller should call @c data->free(data) to dispose of the
+ *         poll data.
  */
 static inline uint32_t poll_dec_ref(MXS_POLL_DATA* data)
 {

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;
     }
 };