Partial Issue 174 - Improve handling of multiple session cookies in request

This does not address the matters of cookies with the "Secure" attribute, or remembering the attributes of sent cookies. I'd still like to look into "Secure" attribute cookies a little bit more.

But, even this alone should still greatly improve Vibe.d's handling of the situations where multiple session cookies exist in the request.

It was necessary to change HttpServerRequest.cookies from ```string[string]``` to ```string[][string]```. This is a breaking change, but I don't see a good way around it since, as we discovered, there are situations where receiving multiple cookies of the same name can be expected, and must be handled properly.

I recognize the concern about the overall usefulness of ```cookies``` being ```string[][string]``` since a cookie's attributes are not provided in the request. However, this change *does* make it possible for the user's Vibe.d-based application, or even Vibe.d itself, to keep track of such attributes and associate them with the correct cookies even when the cookies have the same name (Which can occur in situations that are not always preventable).