Add an option to not preserve old data in db_json.

data/anope.example.conf

@@ -1147,8 +1147,20 @@ module
 	 * Allows Anope to continue writing the database even if the original can
 	 * not be backed up. This is not recommended as it may result in total data
 	 * loss during the circumstances described above.
+	 *
+	 * Defaults to no.
 	 */
 	#ignore_backup_failure = yes
+
+	/*
+	 * Whether to preserve unknown data within the database. When a module is
+	 * unloaded its data will be kept in the database so that it continues to
+	 * function when reloaded. You can turn this off to clean up unknown data
+	 * on the next database write.
+	 *
+	 * Defaults to yes.
+	 */
+	#preserve_unknown_data = no
 }
 
 /*

modules/database/db_json.cpp

@@ -363,6 +363,10 @@ public:
 
 	void OnSaveDatabase() override
 	{
+		// Step 0: clear the old data if we don't want to preserve it.
+		if (!Config->GetModule(this).Get<bool>("preserve_unknown_data", "yes"))
+			databases.clear();
+
 		// Step 1: clear the old data.
 		for (const auto &type : Serialize::Type::GetTypeOrder())
 		{