Migrates the lastPostsCache from PostStore into cache layer (#13141)

* Some advances

* Partial advances

* Cache moved

* Tests finished

* Removed all test for PostStore (store/sqlstore) related with Cache. This is tested in the cache layer now

model/cluster_message.go

@@ -27,6 +27,7 @@ const (
 	CLUSTER_EVENT_INVALIDATE_CACHE_FOR_EMOJIS_BY_ID                 = "inv_emojis_by_id"
 	CLUSTER_EVENT_INVALIDATE_CACHE_FOR_EMOJIS_ID_BY_NAME            = "inv_emojis_id_by_name"
 	CLUSTER_EVENT_INVALIDATE_CACHE_FOR_CHANNEL_MEMBER_COUNTS        = "inv_channel_member_counts"
+	CLUSTER_EVENT_INVALIDATE_CACHE_FOR_LAST_POSTS                   = "inv_last_posts"
 	CLUSTER_EVENT_CLEAR_SESSION_CACHE_FOR_ALL_USERS                 = "inv_all_user_sessions"
 	CLUSTER_EVENT_INSTALL_PLUGIN                                    = "install_plugin"
 	CLUSTER_EVENT_REMOVE_PLUGIN                                     = "remove_plugin"