Always store the region of a bucket if it is not cached

getBucketRegion tries to obtain the region from:

1. the regionsOfBuckets
2. if it is not present in the cache, from S3

However, in the second case we are not updating the cache!

While for the asset tree the region is always present in the cache, this is not always true for the delivery endpoint, where we are seeing performance issues. Thus, we need to, when the region is not present in the cache, update it so future requests can use it.