Enhancement: Improve memory and runtime for the s3 backend

We've updated the library used for accessing s3, switched to using a lower
level API and added caching for some requests. This lead to a decrease in
memory usage and a great speedup. In addition, we added benchmark functions for
all backends, so we can track improvements over time. The Continuous
Integration test service we're using (Travis) now runs the s3 backend tests not
only against a Minio server, but also against the Amazon s3 live service, so we
should be notified of any regressions much sooner.

https://github.com/restic/restic/pull/962
https://github.com/restic/restic/pull/960
https://github.com/restic/restic/pull/946
https://github.com/restic/restic/pull/938
https://github.com/restic/restic/pull/883
