Reduce the responsibilities of CompactCommand in favour of Compact

The offline compaction tool frontend has responsibilities that should be moved to the backend. In particular, printing progress and error messages, implementing logic based on the input parameters and the environment, and choosing exit status codes should be implemented by the backend.

While CompactCommand was reasonable when both oak-segment and oak-segment-tar coexisted, the current implementation violates responsibility and makes the tool frontend needlessly complicated.