Function scan_pending_receives 

async fn scan_pending_receives(
    btc_rpc: &ServerBitcoinRpcMonitor,
    pks_hash: &Hash,
    mempool_budget: Option<Duration>,
    cache: &mut PendingCache,
) -> Result<(PendingOutputs, MempoolScan)>

Collects every receive output a guardian can see but the federation has not yet recorded, from the recent blocks and, unless mempool_budget is None because the mempool scan is turned off, the mempool.

Also returns how the mempool pass ended, so the caller can tell whether to continue straight away.