Allow X-Forwarded-For to have multiple values [2.1 backport]

[kassner]

Backport of #8409

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PR #1546 was closed because CLA issues, thus I'm resubmitting the changes.

Preconditions

1. HAProxy/Nginx offloading SSL and sending traffic to Varnish;
2. Varnish in front of Apache/Nginx.

Steps to reproduce

1. Place an order according to the preconditions.

Expected result

1. Current customer IP address stored into quote and order tables.

Actual result

1. Stored IP is Varnish/HAProxy IP address relative to the webserver (usually 127.0.0.1 or some internal IP, depends on network structure);

[magento-team]

@kassner thank you for your contribution to Magento 2 project