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.

sweep/fee_estimator_mock_test.go

@@ -16,6 +16,10 @@ type mockFeeEstimator struct {
 
 	blocksToFee map[uint32]lnwallet.SatPerKWeight
 
+	// A closure that when set is used instead of the
+	// mockFeeEstimator.EstimateFeePerKW method.
+	estimateFeePerKW func(numBlocks uint32) (lnwallet.SatPerKWeight, error)
+
 	lock sync.Mutex
 }
 
@@ -45,6 +49,10 @@ func (e *mockFeeEstimator) EstimateFeePerKW(numBlocks uint32) (
 	e.lock.Lock()
 	defer e.lock.Unlock()
 
+	if e.estimateFeePerKW != nil {
+		return e.estimateFeePerKW(numBlocks)
+	}
+
 	if fee, ok := e.blocksToFee[numBlocks]; ok {
 		return fee, nil
 	}