flush after redirect with conntrack

[meowjesty]

• Issue: flush existing connections on steal #1029

Adds conntrack --flush to the agent whenever stealer redirect rule is added.

The feature is gated behind the flush_connections: bool config, and the MIRRORD_AGENT_STEALER_FLUSH_CONNECTIONS agent env var.

[meowjesty]

Added an env var for this feature, and you can enable it in mirrord-config with the flush_connections: true option.

Have tested it locally (not yet in k3s), and it doesn't seem to be breaking anything.

[aviramha]

Great job! We can merge it.