Cleanup in appropriate order when running parallel execution

[dsa0x]

This PR started with me tracking a bug where in parallel mode, we still had some dependency ordering based on the sequence of runs.

For example,

test {
  parallel = true
}

run "test" {
  state_key = "main"
  variables {
    id = "test"
    unused = "unused"
  }
}

run "test_two" {
  state_key = "state2"
  variables {
    // This dependency is a later run, but that should be fine because we are in parallel mode.
    id = run.test_three.id

    // The output state data for this dependency will also be left behind, but the actual
    // resource will have been destroyed by the cleanup step of test_three.
    unused = run.test.unused
  }
}

run "test_three" {
  state_key = "state3"
  variables {
    id = "test_three"
    unused = run.test.unused
  }
}

run "test_four" {
  state_key = "main"
  variables {
    id = "test"
    unused = "unused"
  }
}

In the test above, run.test_three is destroyed before run.test_two, but that is wrong. test_two depends on test_three, so test_two should be destroyed first, i.e reversing the order of creation.

Having fixed that in f6f234f, I realized that the original implementation was unnecessarily complex. Instead, what I have done now is only connect references between the last runs of each state key. These last runs represent the last applied state.

In the above, the state_key main will contain the state applied after evaluating run.test_four. Therefore, any reference to run.test (which also used the state_key main) do not need to be taken into account when connecting cleanup nodes, as run.test_four is the "owner" of the cleanup process.

Target Release

1.15.x

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.