lnwallet: add configurable cache for web fee estimator

Add fee.min-update-timeout and fee.max-update-timeout config options to
allow configuration of the web fee estimator cache.

config.go

@@ -350,7 +350,7 @@ type Config struct {
 	MaxPendingChannels int    `long:"maxpendingchannels" description:"The maximum number of incoming pending channels permitted per peer."`
 	BackupFilePath     string `long:"backupfilepath" description:"The target location of the channel backup file"`
 
-	FeeURL string `long:"feeurl" description:"Optional URL for external fee estimation. If no URL is specified, the method for fee estimation will depend on the chosen backend and network. Must be set for neutrino on mainnet."`
+	FeeURL string `long:"feeurl" description:"DEPRECATED: Use 'fee.url' option. Optional URL for external fee estimation. If no URL is specified, the method for fee estimation will depend on the chosen backend and network. Must be set for neutrino on mainnet." hidden:"true"`
 
 	Bitcoin      *lncfg.Chain    `group:"Bitcoin" namespace:"bitcoin"`
 	BtcdMode     *lncfg.Btcd     `group:"btcd" namespace:"btcd"`
@@ -442,6 +442,8 @@ type Config struct {
 
 	DustThreshold uint64 `long:"dust-threshold" description:"Sets the dust sum threshold in satoshis for a channel after which dust HTLC's will be failed."`
 
+	Fee *lncfg.Fee `group:"fee" namespace:"fee"`
+
 	Invoices *lncfg.Invoices `group:"invoices" namespace:"invoices"`
 
 	Routing *lncfg.Routing `group:"routing" namespace:"routing"`
@@ -582,6 +584,12 @@ func DefaultConfig() Config {
 		MinBackoff:         defaultMinBackoff,
 		MaxBackoff:         defaultMaxBackoff,
 		ConnectionTimeout:  tor.DefaultConnTimeout,
+
+		Fee: &lncfg.Fee{
+			MinUpdateTimeout: lncfg.DefaultMinUpdateTimeout,
+			MaxUpdateTimeout: lncfg.DefaultMaxUpdateTimeout,
+		},
+
 		SubRPCServers: &subRPCServerConfigs{
 			SignRPC:   &signrpc.Config{},
 			RouterRPC: routerrpc.DefaultConfig(),