Make routingworker.hh public

Given that worker.hh was public, it made sense to make routingworker.hh
public as well. This removes the need to include private headers in
modules and allows C++ constructs to be used in C++ code when previously
only the C API was available.

server/core/test/test_utils.h

@@ -24,11 +24,11 @@
 #include <maxscale/query_classifier.h>
 #include <maxscale/paths.h>
 #include <maxscale/alloc.h>
+#include <maxscale/routingworker.hh>
 
 #include <sys/stat.h>
 
 #include "../internal/poll.hh"
-#include "../internal/routingworker.hh"
 #include "../internal/statistics.h"