Speedup

[JakubOnderka]

This change speedups notify_push Rust application:

• Uses ahash also creating hash for user ID or for storage mapping HashMap
• Uses PassthruHasher for already hashed values, like UserId
• Mark some methods as inline
• Remove closed connections from active connections hash map
• Add active user count to metrics

It also updates dependencies, bumps minimal Rust version to 1.56 and fix some clippy warnings.

[icewind1991]

Hello,

thanks for the interest in contributing, could you split up the changes into separate PRs to make them easier to reason about