Batch MW parser and imageinfo API requests
- Implement a system for mixed batches of parser, preprocessor and imageinfo requests. This uses an MW extension specific to Parsoid which provides the relevant API.
- Implement caching inside Batcher, replacing env.pageCache, except for its original use case. parserTests.js uses env.pageCache to inject template wikitext, which will still work -- it was never really correct to allow parserTests.js to inject other API responses into the cache.
- Remove Processor parameter from fetchExpandedTpl() since it was always the same.