cloud/storage: Remove the obsolete object touching example

The backend rejects insertions of objects with no content.
The call returns ErrObjectNotFound, and avoids to insert
the new object.

Change-Id: I5a6b59f82ad87e3fc71c4a709165e65bd28261d4
Don't make []byte fields automatically "noindex". Remove ByteString.

Change-Id: I2b5f3fcfed2f0aedcde7509b0eb574876bf9e7e8
cloud/storage: Generate signed URLs

Change-Id: I873f2c7514a5920acdfcacfbc712ab7b2c3b4f70