Rename RemoteAddress::getOne to getClientAddress

Summary:
“get one” asks a second question: “which one?”

The answer is the first one, ie “the client IP”, per future RFC 7239.

This is a follow-up for change b7683f8d6cf8.

Test Plan: Run tests to ensure it's correctly renamed everywhere

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1625