Make tfjs-node create a new TFJSBackend for each environment

[bmcdorman]

This fixes issue #4947.

tfjs-node assumes a single global TFJSBackend for all napi_envs. This assumption holds for most applications, but not when embedding node in C++ with multiple environments or when using web workers. The user observes typical non-deterministic behavior with random crashes and weird error messages.

To resolve this, a map is created that holds a TFJSBackend for each napi_env.

Cleanup wasn't handled by the previous global implementation, so it's also not handled by this implementation. This may be undesirable. I also did this quickly to fix an issue I was having with little regard for style, DRYness, or other considerations, so hope it's a useful starting point for a project developer at least.

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This change is 

[pyu10055]

Thank you very much for the contribution. Our team is not specialized in the node.js development, sorry that we missed the clean up logic for the backend instance. We would love to hear more about your thoughts on the backend clean up, how we should hook up with the node napi_env tear down.

Reviewable status: 0 of 1 approvals obtained (waiting on @bmcdorman)

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tfjs-node/binding/tfjs_binding.cc, line 79 at r1 (raw file):

Quoted 7 lines of code…

  TFJSBackend *backend = nullptr;
  {
    std::lock_guard<std::mutex> lock(gBackendsMut);
    const auto it = gBackends.find(env);
    if (it == gBackends.cend()) NAPI_THROW_ERROR(env, "Backend not found for environment");
    backend = it->second;
  }

I will be good to extract this logic into a separate method to reduce code duplication.

[bmcdorman]

It looks like there's actually a better way to go about this with Node's Environment Lifecycle API. That'll remove the lock and also make cleanup trivial to implement. I'll take another stab at it in the next couple days.

Thanks for the feedback!

[bmcdorman]

Okay, cleanup has been added and we're now using the Node Environment Lifecycle API. Removing the lock also sped things up a little for me in my testing (with 3 environments hammering tfjs-node). I think it's good to go.

[pyu10055]

Environment Lifecycle API
Looks great, thanks!

Reviewed 2 of 2 files at r2.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @bmcdorman)