bug: new_default_process_async_with_migrations() calls the synchronous migrate callback from async context

[lf-]

I am using an async migration runner which means that I need to run async code inside the migrate callback. Sure, Runtime::block_on, whatever, but that turns out it blows up because it's already being run on a runtime thread.

This is because the migrate callback is called on an async thread (!), which is certainly a bug. The fix to this is to run it in spawn_blocking and/or switch the callback to be itself asynchronous (which is plausibly a pain in the ass because this stuff always is).

[johnchildren]

Sorry for the slow response, only just saw this now. I've made this PR: #19

I'll try and test it in our own codebase and then hopefully can get a merge and release out.