Sync implementation for boxed queries

[dullbananas]

Boxed queries should implement Sync (not just Send) so that a custom query can contain a reference to it. Because I can't do that, I instead need to make a query with a Drop implementation that moves the subquery into a cloned Arc<Mutex<T>> so that it can be accessed and used again.

https://docs.rs/diesele/0.1.0/src/diesele/lib.rs.html#302

[weiznich]

It's unfortunately not possible to change this without a breaking change. As we don't plan any new major diesel version bump anytime soon this cannot be implemented as far as I'm aware.