Feature/soledad adbapi3

DO NOT MERGE YET
just for testing / pullreq purposes

As it is right now, i think the content will be converted right away upon `put_doc()` call, before the deferred is even instantiated, right?

The reason for this was originally documented in bug 3660, but that was "archived" and I can't find it. There's a hint about it in https://leap.se/code/issues/4095