multi: cap anchors feerate at configurable maximum

This commit caps the update fee the initiator will send when the anchors
channel type is used. We do not limit anything on the receiver side.

10 sat/vbyte is the current default max fee rate we use. This should be
enough to ensure propagation before anchoring down the commitment
transaction.

config.go

@@ -33,6 +33,7 @@ import (
 	"github.com/lightningnetwork/lnd/lncfg"
 	"github.com/lightningnetwork/lnd/lnrpc/routerrpc"
 	"github.com/lightningnetwork/lnd/lnrpc/signrpc"
+	"github.com/lightningnetwork/lnd/lnwallet"
 	"github.com/lightningnetwork/lnd/routing"
 	"github.com/lightningnetwork/lnd/tor"
 )
@@ -289,6 +290,8 @@ type Config struct {
 
 	MaxChannelFeeAllocation float64 `long:"max-channel-fee-allocation" description:"The maximum percentage of total funds that can be allocated to a channel's commitment fee. This only applies for the initiator of the channel. Valid values are within [0.1, 1]."`
 
+	MaxCommitFeeRateAnchors uint64 `long:"max-commit-fee-rate-anchors" description:"The maximum fee rate in sat/vbyte that will be used for commitments of channels of the anchors type. Must be large enough to ensure transaction propagation"`
+
 	DryRunMigration bool `long:"dry-run-migration" description:"If true, lnd will abort committing a migration if it would otherwise have been successful. This leaves the database unmodified, and still compatible with the previously active version of lnd."`
 
 	net tor.Net
@@ -475,6 +478,7 @@ func DefaultConfig() Config {
 		},
 		MaxOutgoingCltvExpiry:   htlcswitch.DefaultMaxOutgoingCltvExpiry,
 		MaxChannelFeeAllocation: htlcswitch.DefaultMaxLinkFeeAllocation,
+		MaxCommitFeeRateAnchors: lnwallet.DefaultAnchorsCommitMaxFeeRateSatPerVByte,
 		LogWriter:               build.NewRotatingLogWriter(),
 		DB:                      lncfg.DefaultDB(),
 		registeredChains:        chainreg.NewChainRegistry(),
@@ -735,6 +739,12 @@ func ValidateConfig(cfg Config, usageMessage string) (*Config, error) {
 			cfg.MaxChannelFeeAllocation)
 	}
 
+	if cfg.MaxCommitFeeRateAnchors < 1 {
+		return nil, fmt.Errorf("invalid max commit fee rate anchors: "+
+			"%v, must be at least 1 sat/vbyte",
+			cfg.MaxCommitFeeRateAnchors)
+	}
+
 	// Validate the Tor config parameters.
 	socks, err := lncfg.ParseAddressString(
 		cfg.Tor.SOCKS, strconv.Itoa(defaultTorSOCKSPort),