[RLlib] Fix PR 16162: Having added sleep to _NextValueNotReady causes TD3 tests to become flakey.

[sven1977]

This PR offers a fix for PR 16162's problem with causing TD3 tests to fail/timeout often.
PR 16162 added a sleep to _NextValueNotReady causes TD3 tests to become flakey.

• This PR removes that sleep from the c'tor of _NextValueNotReady, but keeps the sleep in place inside IMPALA's learner_thread such that the original issue should remain fixed.
• See also this discussions here: https://discuss.ray.io/t/very-slow-gradient-descent-on-remote-workers/1278/9

Why are these changes needed?

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 :(