Refactor the setting to make unit test easier (#22405)

Some bugs caused by less unit tests in fundamental packages. This PR
refactor `setting` package so that create a unit test will be easier
than before.

- All `LoadFromXXX` files has been splited as two functions, one is
`InitProviderFromXXX` and `LoadCommonSettings`. The first functions will
only include the code to create or new a ini file. The second function
will load common settings.
- It also renames all functions in setting from `newXXXService` to
`loadXXXSetting` or `loadXXXFrom` to make the function name less
confusing.
- Move `XORMLog` to `SQLLog` because it's a better name for that.

Maybe we should finally move these `loadXXXSetting` into the `XXXInit`
function? Any idea?

---------

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: delvh <dev.lh@web.de>

routers/private/manager.go

@@ -116,11 +116,11 @@ func AddLogger(ctx *context.PrivateContext) {
 	}
 
 	if _, ok := opts.Config["level"]; !ok {
-		opts.Config["level"] = setting.LogLevel
+		opts.Config["level"] = setting.Log.Level
 	}
 
 	if _, ok := opts.Config["stacktraceLevel"]; !ok {
-		opts.Config["stacktraceLevel"] = setting.StacktraceLogLevel
+		opts.Config["stacktraceLevel"] = setting.Log.StacktraceLogLevel
 	}
 
 	if opts.Mode == "file" {
@@ -135,7 +135,7 @@ func AddLogger(ctx *context.PrivateContext) {
 		}
 	}
 
-	bufferLen := setting.Cfg.Section("log").Key("BUFFER_LEN").MustInt64(10000)
+	bufferLen := setting.Log.BufferLength
 	byteConfig, err := json.Marshal(opts.Config)
 	if err != nil {
 		log.Error("Failed to marshal log configuration: %v %v", opts.Config, err)