[serve] Deprecate client-based API in favor of process-wide singleton

[edoakes]

Why are these changes needed?

Deprecates the Client-based API in favor of directly calling API calls like serve.create_backend(). We made an overcorrection here from the previous API that made things just more verbose/annoying. This change is a good middle ground: users can start temporary clusters attached to the lifetime of their session by calling serve.start() in the code. Otherwise, they can start a long-lived session with serve.start(detached=True). API calls if neither of these are made implicitly call serve.connect() to connect to a long-lived session.

This PR still supports the old API but prints a warning message telling people to change their usage. The nice thing here is all they should need to do is s/client\./serve./g.

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/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
  • Release tests
  • This PR is not tested :(

[architkulkarni]

Looks good!