Optimize single thread instances

[llucax]

Actix-web runs tasks in multiple threads, which forces all (mutable) shared data to be synchronized, which could slow things down (like connection pools). Given that the service should be scalable, and running multiple instances should be supported from day one, it might not be worth paying the extra cost of synchronization if we are going to have multiple processes anyway, being able to run multiple instances in multi-CPU servers to take advantage of all the cores.