bug: piece metadata is not cleaned up in case of dataset termination

This is more of a notice than a need to solve the source issue as piece metadata stops being accessible in #574

Also it adds an argument for permissionless cleanup.

No files or tests are identified. Start by reading issue #574 and tracing what happens to piece metadata when a dataset terminates; this issue currently presents the matter as a notice rather than a defined change. It is not ready for a first contribution until the intended cleanup behavior and completion criteria are specified.

Written by the indexing model from the issue text.