Set timeout for HTTP uploads

[blackandred]

In case, when the server will not be responding we need to set a timeout on client side.

Case:

• Storage backend went down, so the server's readiness was unhealthy
• In meantime the client was uploading, the job hanged for 10 hours

[blackandred]

I suggest also to add a client-side verification of the server before starting the upload. The could be a server endpoint to verify client supplied authorization, write access and possibility to perform the backup at all.

That would be an additional layer of protection against weird cases, and would increase clarity - as the client would be able to know what is going on.

Example messages returned back:

• "The server is unhealthy"
• "Access denied"
• "Not enough disk space" (the last one may not be possible to implement due to backend storage abstraction, but lets check it)

[blackandred]

Related issue #48