A number of methods have their Promises cached, to avoid repeated worker round-trips, since they're expected to be called more than once from the default viewer. The way that the caching is currently implemented means that we need to remember to manually clear these Promises on document cleanup/destruction, and it'd be nice to avoid that.
With this patch the relevant Promises are now instead placed in just one `Map`, which is easy to clear, and a new helper method is also introduced to reduce duplication for *simple* `WorkerTransport` methods.
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