[serve] Use controller namespace when getting actors

[shrekris-anyscale]

Why are these changes needed?

Serve gets actors using the current Ray namespace. However, the Ray namespace and the controller namespace may not match when using the _override_controller_namespace argument in serve.start(). This change ensures that the get_actor() calls in ActorReplicaWrapper use the controller namespace. This also allows num_replicas to be scaled up and down properly when using _override_controller_namespace.

Related issue number

N/A

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/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
    • New unit tests are added to ensure num_replicas scales up and down correctly when using _override_controller_namespace.