Refactor such that metadata and etag are
combined to a single argument `srcInfo`.
This is a precursor change for #5544 making
it easier for us to provide encryption/decryption
functions.
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
Harshavardhana