Cap the redirect_cache size to prevent memory abuse

It'd be easily possible to run out of memory from a long running process and session that does a lot of redirects. This is also potentially able to be abused if some information was known about how a service operated. Relatively unlikely, but it's a detail that users shouldn't have to think about. We can enforce something better.

And caching more than 10,000 redirects, in my opinion, is unreasonable. I'm open to a different value. Both more and less conservative.