[serve] Refactor replica wrapper

[zcin]

Why are these changes needed?

Refactor replica wrapper in replica scheduler such that:

• The class that replica scheduler interacts with is RunningReplica.
• RunningReplica holds references to wrapper classes (e.g. ActorReplicaWrapper) that handle sending the actual request over the wire to the replicas.

Related issue number

Checks

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• 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 added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• 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
  • Release tests
  • This PR is not tested :(

[GeneDer]

LGTM!

[edoakes]

definitely looks like something