Automatically clean activity log database

MediaBrowser.Model/Activity/IActivityManager.cs

@@ -16,5 +16,12 @@ namespace MediaBrowser.Model.Activity
         Task CreateAsync(ActivityLog entry);
 
         Task<QueryResult<ActivityLogEntry>> GetPagedResultAsync(ActivityLogQuery query);
+
+        /// <summary>
+        /// Remove all activity logs before the specified date.
+        /// </summary>
+        /// <param name="startDate">Activity log start date.</param>
+        /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
+        Task CleanAsync(DateTime startDate);
     }
 }