RFR: Use one future [v3]

[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`.

Paul Sandoz has updated the pull request incrementally with one additional commit since the last revision:

  Switch expression style update.
  
  (No support in IntelliJ yet for such reformatting.)

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Changes:
  - all: https://git.openjdk.java.net/loom/pull/144/files
  - new: https://git.openjdk.java.net/loom/pull/144/files/b835d22f..9c7e63bd

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=loom&pr=144&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=loom&pr=144&range=01-02

  Stats: 6 lines in 1 file changed: 0 ins; 0 del; 6 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