fix migration script

[cleanup] useless try except

[source storage] refactor source sql generation and results handling to allow repository side callbacks for instance with the BytesFileSystemStorage, before this change: * fspath, _fsopen function were stored procedures executed on the database -> files had to be available both on the repository *and* the database host * we needed implementation for each handled database Now, those function are python callbacks executed when necessary on the repository side, on data comming from the database. The litle cons are: * you can't do anymore restriction on mapped attributes * you can't write queries which will return in the same rset column some mapped attributes (or not mapped the same way) / some not This seems much acceptable since: * it's much more easy to handle when you start having the db on another host than the repo * BFSS works seemlessly on any backend now * you don't bother that much about the cons (at least in the bfss case): you usually don't do any restriction on Bytes... Bonus points: BFSS is more efficient (no queries under the cover as it was done in the registered procedure) and we have a much nicer/efficient fspath implementation. IMO, that rocks :D

[bfss] fix name error

[source storage] on deletion, entity has no edited_attributes

[cleanup] rename dbms_helper to dbhelper for consistency

[entity] we also have to override dict.pop to properly maintain edited_attributes consistency

[bfss test] some minor refactoring. Test deletion + creation/update/deletion rollback

don't put hooks deleting orphan composites into the 'integrity' category, we usually want it when integrity is deactivated

[facet] allow to specify a target type on relation facets for case such as Keyword/CodeKeyword where we only targets Keyword