Simplify storage and tests code.

Change the database structure so that the "attachment" store contains one entry per attachment.

Add tests for memorystorage.
Drop copy/paste code in jio.post. Should be added in a standalone storage if needed.

All ERP5 callable can be represented as attachment.
Editing (FormData) is done with putAttachment.

It is not possible to directly create/modify.

Contrary to previous implementation, the content is not shared between all memory storages.

Allow to read all entries in localStorage.
localStorage becomes just one jio document with many attachments.

This cache doesn't have any conflict detection and can lead to data lose.

Each webdav file is transformed as a single "enclosure" attachment to a jio document with the same name.

Dav directory is represented as one jio document.
Its files are represented as its attachment.

This allows to start from a non empty webdav (with music, pictures for examples)

Alldocs is not implemented for now but subdirectory can still be created.

Enough for now to force usage of querystorage and simplify allDocs everywhere else.