Refactor architecture to be more threadsafe

[koonpeng]

🎉 New feature

Summary

General outline of the refactor

• A ThreadSafe class to protect access of an object behind a mutex.
  • Consumers must call Lock() to get a reference to the object, the reference does locking with RAII, they are also non copyable and non movable to prevent the reference from "escaping" the lock.
• Mutations happen directly on the usd session layer as updates comes in, all updates are done on the subscription threads. Previously updates are done in a mix of the subscription threads and the main thread.
• No longer uses polling to update the scene, a service request to <world>/scene/info is done at the start to populate the stage, then a subscription to <world>/scene/info and <world>/scene/deletion to update the scene as models are added and removed.
  • As a result, there is no longer a need to resolve the scene changes between each poll request.
• At each tick on the main render thread, the mutations on the session layer is committed to omniverse.

TODO: Test meshes support

Test it

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

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[ahcorde]

should we remove from the entities map the id ?