Use epoll instance of Worker

Now the epoll instance of the Worker is used when polling. The
work is still done in poll.cc and the worker provides the descriptor
and thread id.

The comments of poll_waitevents() have been removed; they were not
accurate anymore so better to let the code speak for itself.

server/core/maxscale/poll.h

@@ -50,7 +50,10 @@ enum poll_message
 void            poll_init();
 //void          poll_finish(); // TODO: Add this.
 
-void            poll_waitevents(struct mxs_worker *worker);
+void            poll_waitevents(int epoll_fd,
+                                int thread_id,
+                                bool (*should_terminate)(void* data),
+                                void* data);
 void            poll_set_maxwait(unsigned int);
 void            poll_set_nonblocking_polls(unsigned int);