Remove serialized from_random object ids in tests

[edoakes]

Why are these changes needed?

ObjectID.from_random is not a supported API so we shouldn't use it in tests. This replaces it with asyncio actors for synchronization.

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://ray.readthedocs.io/en/latest/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failure rates at https://ray-travis-tracker.herokuapp.com/.

[AmplabJenkins]

Can one of the admins verify this patch?

[stephanie-wang]

This is great! It'd be good to move the class into utils like Simon said, but the tests look good.

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