- Fix typo error introduced in next_unused_id mysql query
- add a new parametric option to specify custom mysql namedprops data
  path + update documentation and existing test to reflect this change
- add new fixture functions for mysql namedprops test suite
- add test to check next_unused_id
Merge branch 'jkerihuel/indexing-sql' of https://github.com/openchange/openchange into jkerihuel/indexing-sql