Closes#2304
Adds `versitygw utils convert-posix-dataset` (alias `cpd`), which makes a posix dataset not created by the gateway fully compatible with the posix backend. Every top level directory is treated as a bucket and gets a private ACL owned by `--access-key-id` (defaults to the root `--access`) and `BucketOwnerEnforced` object ownership; root level files are ignored. Metadata is stored in xattrs, or in `--sidecar-dir` when set.
`--calculate-etag` and `--checksum-algorithm` optionally compute the MD5 ETag and the `FULL_OBJECT` checksum of every object in a single streamed read. Objects are converted concurrently, tunable with `--concurrency` and `--read-buffer-size`. Existing bucket and object metadata is never overwritten.
niksis02
Ben McClelland