MXS-1929: Split housekeeper initialization

The initialization and starting of the housekeeper is now done
separately. This allows housekeeper tasks to be created when the services
are being created while still preventing the execution of the task before
the startup is complete.

include/maxscale/housekeeper.h

@@ -44,6 +44,13 @@ typedef bool (*TASKFN)(void *data);
  */
 bool hkinit();
 
+/**
+ * Start the housekeeper thread
+ *
+ * @return True if the housekeeper mechanism was started
+ */
+bool hkstart();
+
 /**
  * Waits for the housekeeper thread to finish.
  */