isolate object update logic from container PUT

To further extend pluggble container backends we need to have better interface
for updating object info in a container DB.
Each object has `name`, `timestamp` and metadata, the metadata is taken from the request headers, usually
The new `object_update()` method allows us to isolate metadata extraction.
We can push more logic there if backend supports smarter PUTs (more metadata, for example).

Change-Id: I1aae3e73a56c3800ccd7a8d4f818c518b5118bb1