etcd client for Rust

This is an etcd(API v3) client for Rust, which is refactored from etcd-rs project.

Road Map

• 0.1 Replace the tokio and tonic library with smol and grpc-rs, which are more lightweight libraries.
• 0.2 Refactor etcd api to use grpc-rs async APIs to provide real async functionalities.
• 0.3 Apply the single key-value lock free Cache to etcd client, which is based on the etcd watch mechanism.
• 0.4 Apply the lru replacement policies with lock to etcd Cache.
• 0.5 Add the retry with exponential backoff mechanism to etcd client.
• 0.6 Support the lock free lru replacement policies to etcd Cache.
• 0.7 Support range key-value Cache.