This change brings public data-types such that
we can ask projects to implement gateway projects
externally than maintaining in our repo.
All publicly exported structs are maintained in object-api-datatypes.go
completePart --> CompletePart
uploadMetadata --> MultipartInfo
All other exported errors are at object-api-errors.go
Current implementation didn't honor quorum properly and didn't
handle the errors generated properly. This patch addresses that
and also moves common code `cleanupMultipartUploads` into xl
specific private function.
Fixes#3665