an app is staged when its droplet is uploaded

This removes the requirement that the Stager respond to an individual CC
to complete staging. For now we still send CC-specific URIs, though.

The upload endpoint currently uploads to S3 for the duration of the
request. We may convert this into a background job later.

The second request is still directed at one CC and triggers starting the
other instances, but it is not crucial to the staging having completed.

Introduced Droplet model to simplify the droplet blobstore logic. Not
fully integrated yet.
Gradually killing DSL magics.
Remove in memory state in CC when DEA uploads droplets and buildpack cache.
Clarify blobstore methods
Move logic of URL generations out of StagingsController.
Allow any user with right basic auth credentials to download builbpacks.
It used to only allow admin to download. 
Also allow to skip authentication on per operation basis.

[#56723198]
Refactor droplets URL generation
Rename service broker registration to managedment
Consolidate testing of broker error propagation
Rename StructuredError#source to #error

V1 gateways expose the source as error, not source.
Bump vcap_common