sweep: deepen the interface Aggregator

This commit makes the `ClusterInputs` directly returning the `InputSet`
so the sweeper doesn't know about the existence of `Cluster` interface.
This way we can have a deeper interface as the sweeper only needs to
interact with `Aggregator` only to get the final input sets, leaving the
implementation details being managed by `SimpleAggregator` and future
aggregators.

server.go

@@ -1065,6 +1065,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 
 	aggregator := sweep.NewSimpleUtxoAggregator(
 		cc.FeeEstimator, cfg.Sweeper.MaxFeeRate.FeePerKWeight(),
+		sweep.DefaultMaxInputsPerTx,
 	)
 
 	s.sweeper = sweep.New(&sweep.UtxoSweeperConfig{