v3.18.2

• Refactored balancers (makes concurrent-safe)
• Excluded separate balancers lock from cluster
• Refactored cluster.Cluster interface (Insert and Remove returning nothing now)
• Replaced unsafe cluster.close boolean flag to cluster.done chan for listening close event
• Added internal checker cluster.isClosed() for check cluster state
• Extracted getting available conn from balancer to internal helper cluster.get (called inside cluster.Get as last effort)
• Added checking conn.Conn availability with conn.Ping() in prefer nodeID case

Full Changelog: v3.18.1...v3.18.2