Public API issues

[y-lohse]

@ptbrowne and I had a little chat regarding #241 — we agreed to leave #241 as it is, but found some problems in the current architecture that will have to be addressed at some point. Here's a recap, feel free to add to it / correct me. It's also not very structured.

Having bypass functions is a good thing, but we should avoid using APIs that are considered private. So when here we write query.client.collection(doctype).upload(), we know way too much about the internals of cozy-client.

Ultimately, most of the API is implemented in the collections (DocumentCollection, FileCollection, etc). These are not actually collections, they are just API wrappers. But they are also hard to use at the moment, since they aren't directly exposed.
Even if they were, they need a CozyStackClient instance to work, and it's not trivial to get one of these instances, for exemple from inside a query or a mutation. There is also confusion between CozyStackClient and CozyClient, so renaming one of them might be a good idea.