Rust-Actix

[ingalls]

Context

Rocket has served us very well over the past couple years and I'm a little sad to move away from it's awesome ergonomics.

However in the past year actix has come ahead as a clear contender in the space and I think we should migrate as soon as possible

• Remove dependency on rust nightly (huge)
• Move to futures based connections (more concurrency)
• Move to tokio-postgres to avoid blocking workers

cc/ @mapbox/search