[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

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