add usage of scoped futures to transactions: ownership of this change transferred to @weiznich

[treysidechain]

Hi there! I've been building on top of the diesel-async library for some time and have wanted to be able to use references in the closures I pass to AsyncConnection's transaction method. I wrote up a small crate that encapsulates the behavior needed to support references in the closure, basically it imposes an upper bound on the higher-ranked lifetime that's implicit in the FnOnce trait bound so that it doesn't imply a 'static bound.

Main downside of merging this would be pushing a new function scope_boxed onto code calling transaction, however, Box::pinned still works with the proposed changes.

Here is an example showing how the updated transaction signature allows callback closures to capture references.

[weiznich]

Thanks for opening this PR, this looks great 👍
I would like to reexport the ScopedFutureExt trait from diesel_async so that it is easier for users to import this trait.

In addition: I potentially plan to relicense diesel-async in the future. Because of that I need an explicitly written consent that you transfer the ownership of this change to me. Please add that to the commit message.

[treysidechain]

sure thing, I updated the PR title. If that's not what you're looking for let me know