Fixes#1909
Previously, the mapping between object metadata and posix object was as follows: for each metadata key, we stored a separate xattr with the `user.X-Amz-Meta.<key>` prefix. This resulted in syscall overhead when storing and deleting large numbers of metadata keys.
In addition, very long metadata keys caused failures because most posix filesystems limit xattr key lengths to 127–255 bytes, while S3 does not enforce such a per-key limit.
The logic has now been changed so that all object metadata is stored in a single xattr, `user.metadata`, as a JSON key/value object. For backward compatibility, metadata GET operations still fall back to the old mechanism (`metadata key -> xattr key`) when `user.metadata` is not present.
A new CLI utility has been added to convert all legacy object metadata to the new metadata format within the provided directory.
**Example usage:**
```
versitygw utils convert-xattr-metadata path/to/bucket
```
or
```
versitygw utils cxm path/to/bucket
```
It is recommended to run this command on bucket directories to convert all legacy metadata for every object in the bucket.
97bb70509f