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/memory.go

@@ -87,11 +87,6 @@ func (ms *memoryStore) Load() (err error) {
 	return nil
 }
 
-// Save does nothing, as there is no backing store.
-func (ms *memoryStore) Save() error {
-	return nil
-}
-
 // String returns a hard-coded description, as there is no backing store.
 func (ms *memoryStore) String() string {
 	return "mock://"