MM-12849 Moving all non request scoped items to Server struct (#9806)

* Moving goroutine pool

* Auto refactor

* Moving plugins.

* Auto refactor

* Moving fields to server

* Auto refactor

* Removing siteurl duplication.

* Moving reset of app fields

* Auto refactor

* Formatting

* Moving niling of Server to after last use

* Fixing unit tests.

app/command_echo.go

@@ -78,7 +78,7 @@ func (me *EchoProvider) DoCommand(a *App, args *model.CommandArgs, message strin
 	}
 
 	echoSem <- true
-	a.Go(func() {
+	a.Srv.Go(func() {
 		defer func() { <-echoSem }()
 		post := &model.Post{}
 		post.ChannelId = args.ChannelId