Well, I don't exactly know your scope but I was doing stuff the other day with sqlite and using it on a single thread while having a multi-thread webserver. My solution was to have a wrapper that's "multithreaded" and it'd just accept a `FnMut(transaction) -> Result<T,DbError> + Send + Sync + static` and queue them internally so they ran one at a time. Since all the lambdas are "hardcoded" (like the rest of my source code) the static requirement isn't a big deal.

My main problem with it is that I ended up de/serializing T across the FnMut boundary. But given that the FnMut knows what T is it seems like there should be some way to use unsafe and pointers to remove the serialization if it's ever a performance problem.