BFLD_STRING shall store cached string length internally
The UBF/FML buffer format shall be changed so that:
- For strings we also shall store the length of the string, otherwise when iterating over it requires each time string field to check the length with strlen(), which is not very optimal. If there is hundreds or thousands of string fields, access to last fields becomes time consuming.
WARNING ! After this change any stored binary format UBF buffer becomes invalid. Thus before upgrade, please ensure that all binary UBF buffer are processed.