cloud/storage: prevent nil pointer panic on unset owner field

Change-Id: I05a786cbb7029633d33084d0b8a604c87ce97b0f
cloud: add Péter Szilágyi to contributors

Change-Id: I572a154f84b083ae0933ce119dc00e162fdd53c6
cloud/storage: introduce a new type for mutable object fields

Silently ignoring the dirty read-only Object fields during
put, copy and write is bad practice. Introduce a new type
to limit the user to provide mutable attributes for these calls.

Change-Id: I5917da5a02369846e9a5575e9427cc725f207c65