Use parking_lot for synchronization

[loafofpiecrust]

I use parking_lot for synchronization primitives and it's fantastic compared to std: faster, smaller, and locks don't poison (so you don't need .unwrap() or .expect() on lock functions). I suspect this would bring performance closer to tokio, especially since I think they'll beat tokio's async RwLock since you don't need to use it across an await boundary.

[austinjones]

Thanks for the idea! I've implemented it and it will be going out in v0.5.0 soon.