MXS-1754 Rename mxs::Worker to mxs::RoutingWorker

A new class mxs::Worker will be introduced and mxs::RoutingWorker
will be inherited from that. mxs::Worker will basically only be a
thread with a message-loop.

Once available, all current non-worker threads (but the one
implicitly created by microhttpd) can be creating by inheriting
from that; in practice that means the housekeeping thread, all
monitor threads and possibly the logging thread.

The benefit of this arrangement is that there then will be a general
mechanism for cross thread communication without having to use any
shared data structures.

server/core/session.cc

@@ -41,9 +41,9 @@
 #include <maxscale/protocol/mysql.h>
 
 #include "internal/dcb.h"
-#include "internal/session.h"
 #include "internal/filter.h"
-#include "internal/worker.hh"
+#include "internal/routingworker.hh"
+#include "internal/session.h"
 #include "internal/workertask.hh"
 
 using std::string;