Function start 

pub fn start<B: BlockT, S>(
    client: Arc<dyn BlockBackend<B> + Send + Sync>,
    sync: &S,
    litep2p_handle: BitswapHandle,
    metrics_registry: Option<&Registry>,
) -> (Pin<Box<dyn Future<Output = ()> + Send>>, BitswapHandle)
where
    S: SyncEventStream + ?Sized,

Build the Bitswap service, returning the service future and the user-facing handle.