Fix tiered storage disconnect between WT_BUCKET_STORAGE and customize_file_system

There is a bit of a disconnect between the WT_BUCKET_STORAGE structure and the local store customize_file_system. They are both acting as a source of information and, in particular, the file system is missing the bucket prefix. They also duplicate a lot of information.

This isn't terribly surprising as they were coded at about the same time, but bottom up and top down and we both needed those things as we went along.

This ticket should figure out if we can have one data structure and one copy of the information.