futures: fix behaviour when executors don't exec callback

futures: fix behaviour when executors don't exec callback

Summary:
When future callbacks are to be executed by an executor (via `via`)
and the executor does not actually execute the callback function
(for whatever reason), then waiting for the final future (the one
returned by `via`) block forever. In case the callback function
that got passed to the executor gets destroyed without being executed,
the future should be set to a folly::BrokenPromise exception instead
of remaining unset forever.

This diff modifies the reference counting in folly::detail::Core
to make sure the reference held by the callback function is
properly removed not only after the callback gets executed, but
also when the callback is destroyed without having been executed.

Reviewed By: yfeldblum

Differential Revision: D3455931

fbshipit-source-id: debb6f3563384a658d1e0149a4aadbbcb268938c