RFR: Use one future

[Paul Sandoz]

Here's an example of changing the structured task scope impls, using one `Future` field and comparing future states. In effect the state can only increase monotonically (as per the order determined by the comparator).

The single future field could be made non-volatile, using the `VarHandle` for the volatile read in `handleComplete` since the happens-before edge should ensure the future is visible to the reading thread after `join`.

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Commit messages:
 - Use one future

Changes: https://git.openjdk.java.net/loom/pull/144/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=144&range=00
  Stats: 133 lines in 1 file changed: 40 ins; 32 del; 61 mod
  Patch: https://git.openjdk.java.net/loom/pull/144.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/144/head:pull/144

PR: https://git.openjdk.java.net/loom/pull/144