[core] When reconstruction is enabled, pin objects created by ray.put()

[stephanie-wang]

Why are these changes needed?

When lineage reconstruction is enabled, Ray will attempt to recreate objects that were stored in plasma and lost due to a failure through task re-execution. However, this will not work if the task depends on an object created by ray.put, since the caller would also have to be restarted.

This PR pins objects that are created by ray.put so that they will not be evicted until all downstream tasks can not be executed again. This guarantees that the object value will be available if lineage reconstruction is needed.

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/latest/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failure rates at https://ray-travis-tracker.herokuapp.com/.
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested (please justify below)

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