MXS-1848 Move Worker from internal to public include dir

maxscale::Worker needs to be public if monitors should be
implementable using it.

server/core/test/test_dcb.cc

@@ -36,8 +36,8 @@
 
 #include <maxscale/config.h>
 #include <maxscale/listener.h>
+#include <maxscale/messagequeue.hh>
 
-#include "../internal/messagequeue.hh"
 #include "../internal/routingworker.hh"
 #include "../dcb.cc"
 #include "test_utils.h"

server/core/test/test_worker.cc

@@ -12,7 +12,8 @@
  */
 
 #include <iostream>
-#include "../internal/worker.hh"
+#include <maxscale/worker.hh>
+#include "../internal/poll.h"
 
 using namespace std;