MM-11434 Performance improvements for post metadata (#9849)

* Remove unused error return value from PreparePostForClient

* Remove unused error return value from PreparePostListForClient

* MM-11434 Parallelize PreparePostListForClient

* MM-11434 Skip looking reactions and files on post whenever possible

* Add note about the use of deprecated fields

app/reaction.go

@@ -6,7 +6,6 @@ package app
 import (
 	"net/http"
 
-	"github.com/mattermost/mattermost-server/mlog"
 	"github.com/mattermost/mattermost-server/model"
 )
 
@@ -112,13 +111,10 @@ func (a *App) sendReactionEvent(event string, reaction *model.Reaction, post *mo
 	message.Add("reaction", reaction.ToJson())
 	a.Publish(message)
 
-	clientPost, err := a.PreparePostForClient(post)
-	if err != nil {
-		mlog.Error("Failed to prepare new post for client after reaction", mlog.Any("err", err))
-	}
+	post.HasReactions = hasReactions
+	post.UpdateAt = model.GetMillis()
 
-	clientPost.HasReactions = hasReactions
-	clientPost.UpdateAt = model.GetMillis()
+	clientPost := a.PreparePostForClient(post)
 
 	umessage := model.NewWebSocketEvent(model.WEBSOCKET_EVENT_POST_EDITED, "", post.ChannelId, "", nil)
 	umessage.Add("post", clientPost.ToJson())