Move I/O calls to the blocking threadpool

[ethowitz]

Currently, the verify_token, get_or_create_user, and allocate_user function calls all run on the worker threads Actix initializes to handle requests. This means that, when these calls are waiting for I/O, they are blocking other work from happening. This wouldn't be as much of an issue if Actix spawned many worker threads, but normally, there are only as many workers as there are CPUs on the host. The best solution to this problem is to move these long-running I/O tasks to the blocking threadpool via block, where there are many more threads available to handle concurrent I/O tasks.