relink: use context manager for lock management

censor: use context manager for lock management

templater: remove redundant member variables from templater class We no longer need them since the engine is instantiated in __init__().

templater: resurrect cache of engine instance The engine-level cache was effectively disabled at 48289eafb37d "templater: drop extension point of engine classes (API)" by mistake, which made template rendering quite slow. Spotted by Martin von Zweigbergk.