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.

lntest/fee_service.go

@@ -17,7 +17,7 @@ import (
 
 // WebFeeService defines an interface that's used to provide fee estimation
 // service used in the integration tests. It must provide an URL so that a lnd
-// node can be started with the flag `--feeurl` and uses the customized fee
+// node can be started with the flag `--fee.url` and uses the customized fee
 // estimator.
 type WebFeeService interface {
 	// Start starts the service.

lntest/node/config.go

@@ -279,7 +279,7 @@ func (cfg *BaseNodeConfig) GenArgs() []string {
 	}
 
 	if cfg.FeeURL != "" {
-		args = append(args, "--feeurl="+cfg.FeeURL)
+		args = append(args, "--fee.url="+cfg.FeeURL)
 	}
 
 	// Put extra args in the end so the args can be overwritten.