Get remote address

Summary:
Provide a Request::getRemoteAddress() to use instead of
$_SERVER['REMOTE_ADDR'] to be proxy compliant.

Security

The web server is trusted to clean headers and only forward to
the PHP processes the whitelisted headers, with validated values
or with values determines by the server.

This plays very well with scenarii like nginx front-end, nginx back-end
and php-fpm.

Ref T1395

Test Plan: Unit tests are provided, from rKGF

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1395

Differential Revision: https://devcentral.nasqueron.org/D1623