chainfee: make sure web API has feerate info during startup

Previously we may get a floor feerate when calling `EstimateFeePerKW`
due to the ticker is not yet ticked, which gives us an empty fee map.
This is now fixed to always query the API to initialize the fee map
during startup.

lnwallet/chainfee/estimator.go

@@ -816,22 +816,24 @@ func (w *WebAPIEstimator) EstimateFeePerKW(numBlocks uint32) (
 //
 // NOTE: This method is part of the Estimator interface.
 func (w *WebAPIEstimator) Start() error {
+	log.Infof("Starting Web API fee estimator...")
+
+	// During startup we'll query the API to initialize the fee map.
+	w.updateFeeEstimates()
+
 	// No update loop is needed when we don't cache.
 	if w.noCache {
 		return nil
 	}
 
 	var err error
 	w.started.Do(func() {
-		log.Infof("Starting web API fee estimator")
-
 		feeUpdateTimeout := w.randomFeeUpdateTimeout()
 
 		log.Infof("Web API fee estimator using update timeout of %v",
 			feeUpdateTimeout)
 
 		w.updateFeeTicker = time.NewTicker(feeUpdateTimeout)
-		w.updateFeeEstimates()
 
 		w.wg.Add(1)
 		go w.feeUpdateManager()