fix: kill active connections when user is removed via gRPC

[nightcrawler42]

Summary

When a user is removed through the gRPC API (AlterInbound → RemoveUser), existing connections survive because user validation only happens during the initial handshake. After authentication, the connection runs independently with a captured User pointer — the validator is never consulted again.

This is a real-world problem for panel operators (Marzban, 3x-ui, etc.) who expect disabled/removed users to be disconnected immediately. Currently, the only way to force-disconnect is to restart the entire xray process, which drops all connections.

Changes

• New ConnTracker (proxy/conntrack.go): per-user connection tracking using atomic sequence IDs and sync.Mutex. Stores both context.CancelFunc and io.Closer (the underlying net.Conn) for each tracked connection.

• KillAll(email): cancels contexts AND closes TCP connections. Context cancellation alone is insufficient because buf.Copy (the core data relay loop in common/buf/copy.go) has no context awareness — it only stops on read/write errors. Closing the net.Conn forces an immediate TCP RST.

• All 5 protocol handlers updated: VLESS, VMess, Trojan, Shadowsocks, Shadowsocks 2022

  • Track() called after user authentication in Process()
  • KillAll() called in RemoveUser() after successful removal

• Shadowsocks 2022 fixes:

  • Bounds check for user index in NewConnection/NewPacketConnection (prevents panic when user removed during active handoff)
  • Error propagation for UpdateUsersWithPasswords in RemoveUser (was silently discarded)

Why not just cancel the context?

buf.Copy loops forever on reader.ReadMultiBuffer() with no ctx.Done() check. Long-lived streams (e.g. Psiphon tunnels, persistent WebSocket connections) survive context cancellation indefinitely. Force-closing the TCP connection is the only reliable way to break all in-flight relay loops.

Testing

• Builds cleanly: CGO_ENABLED=0 go build ./...
• Tested in production with Marzban panel + multiple nodes
• Verified with Psiphon (long-lived tunneled connections) — connections now terminate within seconds of user removal

[Fangliding]

dup #5844