Manta has the ability to allow users to authenticate with a
username other than the main account. We want to expose
this functionality to minio manta gateway.
- Use it to send the Content-MD5 header correctly encoded to S3
Gateway
- Fixes a bug in PutObject (including anonymous PutObject) and
PutObjectPart with S3 Gateway found when testing with Mint.
Manta is an Object Storage by [Joyent](https://www.joyent.com/)
This PR adds initial support for Manta. It is intended as non-production
ready so that feedback can be obtained.
This PR allows 'minio update' to not only shows update banner
but also allows for in-place upgrades.
Updates are done safely by validating the downloaded
sha256 of the binary.
Fixes#4781
