RFR: Use one future [v2]

[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:

  Rename method.

-------------

Changes:
  - all: https://git.openjdk.java.net/loom/pull/144/files
  - new: https://git.openjdk.java.net/loom/pull/144/files/005daa06..b835d22f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=loom&pr=144&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=loom&pr=144&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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