Fix EZP-21585: ORA-12899 when creating content in japanese

Link: https://jira.ez.no/browse/EZP-21585

This fix is not finished, but feel free to check it out.

### Description
Externalize some string related operations in the eZDBInterface
so it is possible to override them to have a different
behavior depending on the DB in use.

### Tests
Manual tests

### TODO
- [x] Open the related ezoracle pull request
- [x] Test for regression on mysql (my env is currently on oracle)