Rewrite stale_slaves

The behavior of mariadbmon was changed so that it better understands
slaves attempting to replicate. Rewrote the test to accommodate the change
in behavior and take the opportunity to use newer code.

maxscale-system-test/mariadb_func.h

@@ -295,6 +295,12 @@ public:
         return get_row(m_conn, q);
     }
 
+    std::string field(std::string q)
+    {
+        Row r = get_row(m_conn, q);
+        return r.empty() ? std::string() : r[0];
+    }
+
     const char* error() const
     {
         return mysql_error(m_conn);