lnd+sweep: move DetermineFeePerKw into config

This commit makes `DetermineFeePerKw` configurable on sweeper so it's
easier to write unit tests for it.

server.go

@@ -1059,10 +1059,11 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 	}
 
 	s.sweeper = sweep.New(&sweep.UtxoSweeperConfig{
-		FeeEstimator:   cc.FeeEstimator,
-		GenSweepScript: newSweepPkScriptGen(cc.Wallet),
-		Signer:         cc.Wallet.Cfg.Signer,
-		Wallet:         newSweeperWallet(cc.Wallet),
+		FeeEstimator:      cc.FeeEstimator,
+		DetermineFeePerKw: sweep.DetermineFeePerKw,
+		GenSweepScript:    newSweepPkScriptGen(cc.Wallet),
+		Signer:            cc.Wallet.Cfg.Signer,
+		Wallet:            newSweeperWallet(cc.Wallet),
 		NewBatchTimer: func() <-chan time.Time {
 			return time.NewTimer(cfg.Sweeper.BatchWindowDuration).C
 		},