Make the AsyncListSum be immutable again, identical performance to its mutable version now in benchmarks.
Reverse the list to ensure we keep it only require associative not commutative (i.e. keep the order)
Merge pull request #309 from twitter/feature/ImmutableAsyncListSum

Make the AsyncListSum be immutable again, identical performance to its m...
Changes for release
Merge branch 'release/0.6.0' into develop