Add MessageQueue class

MessageQueue encapsulates a message queue built on top of a
pipe. The message queue needs a handler for receiving messages
and must be added to a worker for pumping messages through the
pipe.

Each Worker will have an instance of MessageQueue.

include/maxscale/poll_core.hh

@@ -0,0 +1,36 @@
+#pragma once
+/*
+ * Copyright (c) 2016 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl.
+ *
+ * Change Date: 2019-07-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+#include <maxscale/cppdefs.hh>
+#include <maxscale/poll_core.h>
+
+namespace maxscale
+{
+
+struct MxsPollData : MXS_POLL_DATA
+{
+    MxsPollData()
+    {
+        handler = NULL;
+        thread.id = 0;
+    }
+
+    MxsPollData(mxs_poll_handler_t h)
+    {
+        handler = h;
+        thread.id = 0;
+    }
+};
+
+}