(Wangle) Fix Executor problem

(Wangle) Fix Executor problem

Summary:
None of these functions should be templated with `class Executor`.
Except `then(Executor, Args...)` because otherwise the compiler gets
confused. This was the combination that worked for both Clang and GCC,
don't ask me why. I'm assuming this puts it on a low priority...

I think this is also OK, because `setExecutor` takes an actual
`folly::Executor`, so even `then(Executor, Args...)` won't just work for any
`Executor`.

Test Plan: Run all the tests.

Reviewed By: jsedgwick@fb.com

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

FB internal diff: D2036912

Tasks: 6838553

Signature: t1:2036912:1430493088:44f2ffe146298c3f978ac27a45b9b2e33b2b0422