Refactoring of storage service

[pcrespov]

After the upgrade of this service, many improvements seem necesary in order to achieve a maintainable service.

Most of the small issues are just marked as TODO: or FIXME: directly in the code and the most relevant are:

• install blackfynn agent and use it in the code. For the moment, this feature is disabled (look for us_agent=False flag).
• refactor blackfynn wrappers layers dsm -> datcore_wrapper -> dat_core. There is a lot of room for improvement there to make it more maintainable and not so verbose.
• do not use protected api in blackfynn library e.g. bf._api.io.upload
• Prefer None to "" (empty strings) in returns when something
• add annotations

[GitHK]

Both of you should know more about the state of this issue.

[pcrespov]

Outdated and replaced by backlog at #2396