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/database.go

@@ -111,9 +111,10 @@ func parseDSN(dsn string) (string, string, error) {
 	return scheme, dsn, nil
 }
 
-// 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.
 func (ds *DatabaseStore) Set(newCfg *model.Config) (*model.Config, error) {
-	return ds.commonStore.set(newCfg, nil)
+	return ds.commonStore.set(newCfg, nil, ds.persist)
+
 }
 
 // persist writes the configuration to the configured database.
@@ -192,14 +193,6 @@ func (ds *DatabaseStore) Load() (err error) {
 	return ds.commonStore.load(ioutil.NopCloser(bytes.NewReader(configurationData)), needsSave, ds.persist)
 }
 
-// Save writes the current configuration to the backing store.
-func (ds *DatabaseStore) Save() error {
-	ds.configLock.RLock()
-	defer ds.configLock.RUnlock()
-
-	return ds.persist(ds.config)
-}
-
 // String returns the path to the database backing the config, masking the password.
 func (ds *DatabaseStore) String() string {
 	u, _ := url.Parse(ds.originalDsn)