Refactor (blob_id, doc_id, rev) -> (blob_id, FIXED_REV)

local blobs db and server backend doesn't track or even use doc_id and rev, except when user provides it. This makes it hard for the system to download/upload without having this data at hand (only happens during get/put hot calls) and also it looks like redundant since it's not used by inner blobs code.

This refactor will remove them and should unblock a MVP for retry and upload/download error handling.