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.

templater: extract template loader to separate class This avoids reference cycle in the subsequent patch: templater -> _proc -> templater.load -> templater The templater class will be a thin wrapper around the loader and the engine.