multi: support arbitrary client fee preferences to UtxoSweeper

In this commit, we introduce support for arbitrary client fee
preferences when accepting input sweep requests. This is possible with
the addition of fee rate buckets. Fee rate buckets are buckets that
contain inputs with similar fee rates within a specific range, e.g.,
1-10 sat/vbyte, 11-20 sat/vbyte, etc. Having these buckets allows us to
batch and sweep inputs from different clients with similar fee rates
within a single transaction, allowing us to save on chain fees.

With this addition, we can now get rid of the UtxoSweeper's default fee
preference. As of this commit, any clients using the it to sweep inputs
specify the same fee preference to not change their behavior. Each of
these can be fine-tuned later on given their use cases.

contractcourt/commit_sweep_resolver.go

@@ -5,10 +5,16 @@ import (
 	"fmt"
 	"io"
 
-	"github.com/lightningnetwork/lnd/input"
-
 	"github.com/btcsuite/btcd/wire"
+	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/lnwallet"
+	"github.com/lightningnetwork/lnd/sweep"
+)
+
+const (
+	// commitOutputConfTarget is the default confirmation target we'll use
+	// for sweeps of commit outputs that belong to us.
+	commitOutputConfTarget = 6
 )
 
 // commitSweepResolver is a resolver that will attempt to sweep the commitment
@@ -98,7 +104,8 @@ func (c *commitSweepResolver) Resolve() (ContractResolver, error) {
 		// sweeper.
 		log.Infof("%T(%v): sweeping commit output", c, c.chanPoint)
 
-		resultChan, err := c.Sweeper.SweepInput(&inp)
+		feePref := sweep.FeePreference{ConfTarget: commitOutputConfTarget}
+		resultChan, err := c.Sweeper.SweepInput(&inp, feePref)
 		if err != nil {
 			log.Errorf("%T(%v): unable to sweep input: %v",
 				c, c.chanPoint, err)