Optionally run Response Promise to completion in waitUntil context

[kflansburg]

When a client disconnects, code is not run to clean up wasm-bindgen's "heap", and thus objects associated with the request leak. wasm-bindgen suggests the use of FinalizationRegistry, which we do not currently support due to security reasons.

This change allows user's to specify RUN_TO_COMPLETION environment variable when running worker-build, which tricks the Workers runtime into executing the Promise that resolves the Response in a waitUntil context, even if the client disconnects. This should be used with caution, as your code may have side effects that you would not want to run if the client disconnects.

Below is a graph of Wasm heap usage over time during a load test, first with existing behavior, and second with RUN_TO_COMPLETION enabled.