Update to use modern Rust dependencies

[ggrossman]

This sample is 9 years old, and because it doesn't pin versions in Cargo.toml, it no longer compiles out of the box. It also depends on unsupported and deprecated lIbraries such as rustc-serialize.

• nickel web framework replaced with actix-web, which is a modern and actively maintained Rust web framework.
• rustc_serialize crate replaced with serde for serialization and deserialization.
• mongodb crate syntax updated to the latest version, with async/await support.
• jwt crate is replaced with jsonwebtoken, which is a commonly used JWT library in Rust.
• hyper crate and related HTTP utilities are replaced by actix-web features.
• The updated code uses env_logger for logging, actix_web::middleware::Logger for request logging, and futures::StreamExt for MongoDB cursor handling.