TAP5-2832: LocalhostOnly detect IPv6 correctly

[benweidig]

LocalhostOnly didn't detect a few of the IPv6 variants.
Furthermore, it might trigger a DNS lookup, which could impact performance.
Therefore, it now relies on HttpServletRequest.getRemoteAddr (via Request).

I've decided to extend the Request interfrace with getRemoteAddr instead of trying to provide LocalhostOnly with the HttpServletRequest somehow.

My reasoning is that Request is already a more high-level/convenient wrapper for HttpServletRequest, delegating multiple methods already to the underlying HttpServletRequest.
Adding another one makes sense, as anything relying on LocalhostOnly would need to provide the full/correct environment. This was an issue during initial testing with using RequestGlobalsinLocalhostOnly`, as it became an invisible dependency.

By extending Request and the related TestableRequest and DelegatingRequest, the Request-based code will behave correctly, regardless of how it's used.