Get remote address
Provide a Request::getRemoteAddress() to use instead of
$_SERVER['REMOTE_ADDR'] to be proxy compliant.
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
Test Plan: Unit tests are provided, from rKGF
Reviewed By: dereckson
Maniphest Tasks: T1395
Differential Revision: https://devcentral.nasqueron.org/D1623