reverseproxy: Implement cookie selection policy

[d-masson]

Rebase master on @utick branch and try to manage @francislavoie remark on #3408

Fix #3409

[CLAassistant]

All committers have signed the CLA.

[mholt]

Thanks. I'm liking this, but I still need to commit myself to the idea that the load balancer can modify the response. I feel like that should happen separately, but... that might be just as easy to abuse/break... I'll think about it.

[mholt]

Thank you for this! I think I'm finally OK with modifying the response in a selection policy; the user should still be able to control that through configuration so I think it's OK.

Just a few changes requested before we accept this.

[mholt]

LGTM now, thanks so much for your contribution!