MXS-1752: Remove EOF packet generation

The creation of the EOF packet is not needed as the last packet of a
result set is always guaranteed to be of the correct type. This also
allows non-resultsets to be correctly processed as the internal packet
number will be at 0 when the last result arrives.

Cleaned up some of the function names and changed the signatures to be
better suited for their use-cases.

Use angle bracket includes, combine some of the more unwieldly
conditionals into functions, added more comments.

server/modules/routing/cat/catsession.hh

@@ -77,6 +77,10 @@ private:
     mxs::SRWBackendList::iterator m_current;
     GWBUF*                        m_query;
 
-    // Skip over unused backend servers
-    void skip_unused();
-};
+    /**
+     * Iterate to next backend
+     *
+     * @return True if m_current points to a valid backend that is in use
+     */
+    bool next_backend();
+};