Small optimization to container_quotas

When handling an object PUT with X-Copy-From, we only need to fetch
the source object's size if the container has a byte quota, but we
were always fetching it. Now we only fetch the size when we need it.

Change-Id: I0c2c6820cdf0022ef57df5fe7dcb2dd466665a4f
Merge "Small optimization to container_quotas"