Implement S3-like backends

Includes AWS, GCS and Minio (generic S3). Only this last one is actually complete, the others lack Rename() implementations.

Functionality is still completely untested, except for the internal API changes tested in the 'local' source.

Also includes an integration test that dumps cpu/allocs/heap profiles of the ingestion stage under load.

mentioned in commit 1fe7b0b8

merged