Summary:
adds ensureCleanupAfterTask()

takes a SemiFuture<T> task and a SemiFuture<Unit> cleanup.

returns a SemiFuture<T>

always starts cleanup after task completes. stores the result of the task and replays it after the cleanup completes.

cleanup is not allowed to fail.

Reviewed By: yfeldblum, andriigrynenko

Differential Revision: D21481825

fbshipit-source-id: 901a91c25f7b2b4fa794a590a5433e0fad27d168