Function start_pruner 

pub fn start_pruner(
    db: Db,
    config: PrunerConfig,
    metrics: Arc<PrunerMetrics>,
) -> Result<Service>

Start the background pruner as a Service.

Errors if config.retention_epochs is 0 (which would prune the current epoch). The returned service runs an infinite tick loop; it is aborted on graceful shutdown (each chunk is atomic, so an abort leaves the database consistent).