Redis backend based on fred.rs crate

[singulared]

Let us use https://github.com/aembke/fred.rs/ for Redis interaction instead of redis.rs. It has multiple major features:

• Supports clustered, centralized, and sentinel Redis deployments.
• Optional built-in reconnection logic with multiple backoff policies.
• Supports transactions.
• Supports Lua scripts.
• Automatically retry requests under bad network conditions.
• Optional built-in tracking for network latency and payload size metrics.
• An optional client pooling interface to round-robin requests among a pool of clients.
• An optional sentinel client for interacting directly with sentinel nodes to manually failover servers, etc.