eth/ethconfig: flip the default from fast to snap sync

eth/ethconfig/config.go

@@ -57,7 +57,7 @@ var LightClientGPO = gasprice.Config{
 
 // Defaults contains default settings for use on the Ethereum main net.
 var Defaults = Config{
-	SyncMode: downloader.FastSync,
+	SyncMode: downloader.SnapSync,
 	Ethash: ethash.Config{
 		CacheDir:         "ethash",
 		CachesInMem:      2,