Move thread data to workers

This is a step in the direction that any worker/thread related data
is the property of the worker/thread.

server/core/maxscale/poll.h

@@ -52,6 +52,7 @@ void            poll_init();
 
 void            poll_waitevents(int epoll_fd,
                                 int thread_id,
+                                THREAD_DATA* thread_data,
                                 bool (*should_terminate)(void* data),
                                 void* data);
 void            poll_set_maxwait(unsigned int);