(Wangle) Simplify reduce, use the same implementation for func returning T/Future<T>

(Wangle) Simplify reduce, use the same implementation for func returning T/Future<T>

Summary:
I wanted to use `collectAll` for `reduce` if `func` does not return a
Future, because the overhead seemed smaller, but it has been suggested
that running the callback as soon as possible might be better. Not sure
which is. Note that this also makes n copies of the lambda and moves the
value at least n times.

I also plan to add a `streamingReduce` which calls `func` as soon as
results come in (but out of order).

Test Plan: Run all the tests.

Reviewed By: hans@fb.com

Subscribers: folly-diffs@, jsedgwick, yfeldblum, chalfant

FB internal diff: D2015316

Tasks: 6025252

Signature: t1:2015316:1430349611:4f62a540ded85299a22670dd2add27cf1458e8f8