cmd, consensus, core, miner: instatx clique for --dev (#15323)

* cmd, consensus, core, miner: instatx clique for --dev

* cmd, consensus, clique: support configurable --dev block times

* cmd, core: allow --dev to use persistent storage too

cmd/geth/usage.go

@@ -72,7 +72,6 @@ var AppHelpFlagGroups = []flagGroup{
 			utils.NetworkIdFlag,
 			utils.TestnetFlag,
 			utils.RinkebyFlag,
-			utils.DevModeFlag,
 			utils.SyncModeFlag,
 			utils.EthStatsURLFlag,
 			utils.IdentityFlag,
@@ -81,6 +80,12 @@ var AppHelpFlagGroups = []flagGroup{
 			utils.LightKDFFlag,
 		},
 	},
+	{Name: "DEVELOPER CHAIN",
+		Flags: []cli.Flag{
+			utils.DeveloperFlag,
+			utils.DeveloperPeriodFlag,
+		},
+	},
 	{
 		Name: "ETHASH",
 		Flags: []cli.Flag{