[RLlib] Cleanup, rename, clarify: Algorithm.workers/evaluation_workers, local_worker(), etc..

[sven1977]

Renames:
Algorithm.workers -> env_runner_group
Algorithm.evaluation_workers -> evaluation_env_runner_group
EnvRunnerGroup.local_worker -> local_env_runner

Adds shortcuts:
Algorithm.env_runner -> Algorithm.env_runner_group.local_env_runner
Algorithm.eval_env_runner -> Algorithm.evaluation_env_runner_group.local_env_runner

Why are these changes needed?

Related issue number

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• 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 :(

[simonsays1980]

LGTM. One last thing I guess: Now, where the new Offline RL API is merged, we need to apply these renaming also to BC.

[sven1977]

Good point. Done right now through master merge (conflict resolution).