[keepstore] switch s3 driver from goamz to a more actively maintained client library
- official AWS Go SDK, https://github.com/aws/aws-sdk-go. Kubernetes made the switch in May 2015. The Kubernetes PR provides insight into what needs to be changed and possible approaches to migrating the code.
- new/upcoming v2 of official AWS Go SDK, https://github.com/aws/aws-sdk-go-v2
- minio-go, client library from S3-compatible cloud storage system: godoc / API docs ... This might have better compatibility with non-Amazon S3 services like Google cloud storage.
While we will probably choose the official client library when we add Go code that uses EC2, that doesn't necessarily qualify as a reason to choose the official library for storage as well.