Inject clock instead of using process-wide clock

Currently, if we need to get the current time, we use something like Utc::now or SystemTime::now. These use process-wide clocks, which is fine, but which are difficult to mock out for testing (see tests in the cleanup.rs). What we end up doing to test this is mocking out the process-wide clock but this means these tests cannot run in parallel (since they would interfere with each other's mocking).

Instead, we could make a dataset's clock configurable. This would allow us to change it during testing without impacting other running tests.

Start by reading the clock-mocking tests in cleanup.rs and identify how dataset operations currently obtain the process-wide time. Define the dataset-level clock configuration and verify that tests can change it independently, without interfering with parallel tests.