feat: Non-zero Default Fees (cosmos#9371)

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## Description

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closes: cosmos#9106

---

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- [ ] Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
- [ ] Code follows the [module structure standards](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules/structure.md).
- [ ] Wrote unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [ ] Updated relevant documentation (`docs/`) or specification (`x/<module>/spec/`)
- [ ] Added relevant `godoc` [comments](https://blog.golang.org/godoc-documenting-go-code).
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simd/cmd/root.go

@@ -98,8 +98,25 @@ func initAppConfig() (string, interface{}) {
 		WASM WASMConfig `mapstructure:"wasm"`
 	}
 
+	// Optionally allow the chain developer to overwrite the SDK's default
+	// server config.
+	srvCfg := serverconfig.DefaultConfig()
+	// The SDK's default minimum gas price is set to "" (empty value) inside
+	// app.toml. If left empty by validators, the node will halt on startup.
+	// However, the chain developer can set a default app.toml value for their
+	// validators here.
+	//
+	// In summary:
+	// - if you leave srvCfg.MinGasPrices = "", all validators MUST tweak their
+	//   own app.toml config,
+	// - if you set srvCfg.MinGasPrices non-empty, validators CAN tweak their
+	//   own app.toml to override, or use this default value.
+	//
+	// In simapp, we set the min gas prices to 0.
+	srvCfg.MinGasPrices = "0stake"
+
 	customAppConfig := CustomAppConfig{
-		Config: *serverconfig.DefaultConfig(),
+		Config: *srvCfg,
 		WASM: WASMConfig{
 			LruSize:       1,
 			QueryGasLimit: 300000,
@@ -207,7 +224,7 @@ type appCreator struct {
 	encCfg params.EncodingConfig
 }
 
-// newApp is an AppCreator
+// newApp is an appCreator
 func (a appCreator) newApp(logger log.Logger, db dbm.DB, traceStore io.Writer, appOpts servertypes.AppOptions) servertypes.Application {
 	var cache sdk.MultiStorePersistentCache