Increase default metadata key and value size to 2KB

[diwu1989]

AWS S3 seems to have 2KB limit on the metadata key and value.
Swift seems to run with default constraint of 256.

Can we change the default to match 2k?
https://docs.aws.amazon.com/AmazonS3/latest/user-guide/add-object-metadata.html

[timuralp]

I think we need to figure out a few more parameters about AWS S3. Do you know if the maximum key length is limited to 2KB? Is there a limit on the number of x-amz-meta- headers? Swift has a number of settings that we need to tune in concert to match the desired AWS S3 behavior:
https://github.com/openstack/swift/blob/master/swift/common/constraints.py