[MM-44263] Cache Posts Usage (#20254)

* Fetch users only posts

* Cache Posts Usage

* Explicitly validate posts count before testing usage

* Add DocString

* Revert cache changes

* Revert "Revert cache changes"

This reverts commit ba693f8d5028b62c19f6a730e8dca73a224d0761.

model/cluster_message.go

@@ -31,6 +31,7 @@ const (
 	ClusterEventInvalidateCacheForChannelMemberCounts       ClusterEvent = "inv_channel_member_counts"
 	ClusterEventInvalidateCacheForLastPosts                 ClusterEvent = "inv_last_posts"
 	ClusterEventInvalidateCacheForLastPostTime              ClusterEvent = "inv_last_post_time"
+	ClusterEventInvalidateCacheForPostsUsage                ClusterEvent = "inv_posts_usage"
 	ClusterEventInvalidateCacheForTeams                     ClusterEvent = "inv_teams"
 	ClusterEventClearSessionCacheForAllUsers                ClusterEvent = "inv_all_user_sessions"
 	ClusterEventInstallPlugin                               ClusterEvent = "install_plugin"

model/post.go

@@ -281,6 +281,8 @@ type PostCountOptions struct {
 	MustHaveHashtag bool
 	ExcludeDeleted  bool
 	UsersPostsOnly  bool
+	// AllowFromCache looks up cache only when ExcludeDeleted and UsersPostsOnly are true and rest are falsy.
+	AllowFromCache bool
 }
 
 func (o *Post) Etag() string {