s3: route versioned PutObject finalize off the distributed lock
A versioned write's finalize (flip the .versions pointer to the newest version,
demote the prior latest) now runs as a single RECOMPUTE_LATEST ObjectTransaction
on the object's owner filer, under its per-path lock, instead of the unserialized
updateLatestVersionInDirectory. The version file is written first; the owner
re-derives the pointer by scanning the directory.
RECOMPUTE_LATEST gains size_to_key / mtime_to_key to cache the chosen version's
size and mtime on the pointer, and demote_key / demote_value to stamp the
displaced prior latest (NoncurrentSinceNs for lifecycle) when the pointer moves.
Falls back to updateLatestVersionInDirectory when no owner is known yet.