MEDIUM: lb-chash: Deterministic node hashes based on server address

MEDIUM: lb-chash: Deterministic node hashes based on server address

Motivation: When services are discovered through DNS resolution, the order in
which DNS records get resolved and assigned to servers is arbitrary. Therefore,
even though two HAProxy instances using chash balancing might agree that a
particular request should go to server3, it is likely the case that they have
assigned different IPs and ports to the server in that slot.

This patch adds a server option, "hash-key <key>" which can be set to "id" (the
existing behaviour, default), "addr", or "addr-port". By deriving the keys for
the chash tree nodes from a server's address and port we ensure that independent
HAProxy instances will agree on routing decisions. If an address is not known
then the key is derived from the server's puid as it was previously.

When adjusting a server's weight, we now check whether the server's hash has
changed. If it has, we have to remove all its nodes first, since the node keys
will also have to change.