Improve message queue logging

Logging the pipe buffer size on startup will tell how large it was at the
time when MaxScale read it. If there are some abnormalities in it, this
will make it visible.

Logging the worker ID when the posting of a message fails will tell which
particular worker it was. For example, if the worker in question is the
main worker (i.e. ID 0), we know there's something that's blocking the
processing.

include/maxscale/routingworker.hh

@@ -103,7 +103,7 @@ public:
      *
      * @return The id of the routing worker.
      */
-    int id() const
+    int id() const override
     {
         return m_id;
     }