PutBucketVersioning and PutBucketEncryption ran concurrently each did a
whole-entry read-modify-write of the bucket entry, so one could overwrite the
other's field with a stale copy. Each config write is now a field-level
PATCH_EXTENDED (extended attributes) or set_content (the metadata blob)
ObjectTransaction, routed to the bucket's owner filer and merged onto a fresh
read under its per-path lock. Disjoint fields no longer clobber each other.