Expose build_transaction through AsyncConnectionWrapper

[banool]

This addresses #115.

I feel I've made the section in async_connection_wrapper more complex than necessary, I can try tidy it up.

The tests don't pass right now but I know the core section compiles.

Let me know what you think about this solution!

[banool]

Another approach would be to add a function that lets the user call functions inner.

[weiznich]

Thanks for opening this PR. It addresses the problem that there is no build_transaction function for the async connection wrapper type, but I don't feel that the implemented solution is particular help full for down stream crates using that type because the indirectly exposed run function is still async. I would prefer a solution that exposes a sync run method as well. That migth require duplicating or at least modifying parts of TransactionBuilder type, by having a separate run function depending on whether the inner connection type implements diesel::Connection or diesel_async::AsyncConnection.

[banool]

Hmmmm yeah good point. In my case I'm actually already in an async environment so this would work for me, but I realize that that's not the intent of the AsyncConnectionWrapper (since in my case I'd have to use spawn_blocking everywhere). I think I'll just try switch to using diesel-async proper, so I'm going to just close this PR for now. I'll leave the issue open though.