Report the newest member time for object store directory keys

[zaihuaji]

Summary

• check_object_file() reported the timestamp of whichever member the listing happened to return first for a key prefix such as a zarr store. It now reports the newest member's timestamp, which is what the store as a whole was last modified at.
• Version bumped to 3.0.17.

Note

LastModified is zero padded ISO, so a plain max over the listing is chronological. The cnt/isdir computation moved above the object_file_stat() call so the right member can be picked; the rest of the branch is unchanged. Single object keys behave exactly as before.

Side effect: with opt&32 a directory key's checksum now comes from the newest member's ETag rather than the first member's. Neither is meaningful for a store and no current caller asks for a checksum on a directory key.

Test plan

• py_compile clean
• Stubbed listing: directory key returns the newest of three member timestamps, summed size and isfile=0; single object key unchanged
• Rerun dsquasar -A 3 on d694517; expect one round of Timestamp mismatch lines as stored values are corrected