MM-14145: The config store Set will now Save automatically (#10377)

* MM-14145: The config store Set will now Save automatically

When UpdateConfig (and configStore.Set) is called in admin.go and
config.go, commonStore.Set now takes a store-specific persist function.
It uses that persist function to save the configuration automatically.

Removed: Now callers do not have to call configStore.Save or
app.PersistConfig, and those functions have been removed.

Possible downside: this means a "failed to persist config" error can now
be thrown during a app.UpdateConfig or commonStore.Set call. But
considering application code never really sets a config without saving
it (except in the test cases, which were testing that -- see below), it
seems fine to group these responsibilities.

Also removed: tests for 'set without save'. Since that can not happen
anymore, the tests are not needed.

* Removed Save completely, cleaned up formatting, joined save test with
set tests.

* fixed shadowed variable error

config/store.go

@@ -20,15 +20,12 @@ type Store interface {
 	// GetEnvironmentOverrides fetches the configuration fields overridden by environment variables.
 	GetEnvironmentOverrides() map[string]interface{}
 
-	// Set replaces the current configuration in its entirety, without updating the backing store.
+	// Set replaces the current configuration in its entirety and updates the backing store.
 	Set(*model.Config) (*model.Config, error)
 
 	// Load updates the current configuration from the backing store, possibly initializing.
 	Load() (err error)
 
-	// Save writes the current configuration to the backing store.
-	Save() error
-
 	// AddListener adds a callback function to invoke when the configuration is modified.
 	AddListener(listener Listener) string