Update named actor API

[edoakes]

Why are these changes needed?

Updates the ray.util.get_actor and ray.util.register_actor APIs to ray.get_actor and Actor.options(name="name").

This is intentionally not yet surfaced in top-level Ray documentation because it isn't well tested/hardened.

Related issue number

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.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/.
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested (please justify below)

[AmplabJenkins]

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[AmplabJenkins]

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Refer to this link for build results (access rights to CI server needed):
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