Save memory by not storing full request history

[moritz]

See #169.

We used to store the full history of the response, just to count redirects.
This used up lots of RAM for very little gain.

This commit replaces it by a simple integer that stores the count of
redirects in a row.

Since @.history was a public attribute, this changes the public API,
and as such is subject to discussion.

[jonathanstowe]

I'm pretty relaxed about the public API part as history is nether documented nor explicitly tested for (only the re-direct loop detection behaviour is tested,) If, for some reason, there was a need to find the re-direct chain that led to a specific response then this would be better placed in the response (and as the request is already saved in the response, this may be trivial.)

So as long as @ugexe is happy (for it is he who put the redirect limit in originally,) and @sergot doesn't explicitly object, I'll merge this.

[jonathanstowe]

As no-one has raised any objections and in lieu of a better suggestion I am going with this :)