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/common.go

@@ -36,10 +36,11 @@ func (cs *commonStore) GetEnvironmentOverrides() map[string]interface{} {
 	return cs.environmentOverrides
 }
 
-// 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
+// using the persist function argument.
 //
 // This function assumes no lock has been acquired, as it acquires a write lock itself.
-func (cs *commonStore) set(newCfg *model.Config, isValid func(*model.Config) error) (*model.Config, error) {
+func (cs *commonStore) set(newCfg *model.Config, isValid func(*model.Config) error, persist func(*model.Config) error) (*model.Config, error) {
 	cs.configLock.Lock()
 	var unlockOnce sync.Once
 	defer unlockOnce.Do(cs.configLock.Unlock)
@@ -71,12 +72,9 @@ func (cs *commonStore) set(newCfg *model.Config, isValid func(*model.Config) err
 		}
 	}
 
-	// Ideally, Set would persist automatically and abstract this completely away from the
-	// client. Doing so requires a few upstream changes first, so for now an explicit Save()
-	// remains required.
-	// if err := cs.persist(newCfg); err != nil {
-	// 	return nil, errors.Wrap(err, "failed to persist")
-	// }
+	if err := persist(newCfg); err != nil {
+		return nil, errors.Wrap(err, "failed to persist")
+	}
 
 	cs.config = newCfg