allow configuration of multipart cutoff

Currently it's possible that a very large multi-file upload can run a server out of available file handles. 

I realize that setting a specific cut off for everybody is too restricting, but I think it'd be very useful to set a configurable point after which rack will stop handling new parts (and thus creating new tempfiles).

/cc @camilo, @arthurnn, @bslobodin
Using `getrlimit` is indeed a good idea, but what about multithreaded servers? Correct me if I'm wrong but these limit are by process, so we would need to divide the limit by the number of threads.

Or maybe it would just be the user responsibility to do it?

Also the system overcommit the file descriptors. In our case we use unicorn, so no threading, and one process could exhaust all file descriptors of the system, resulting in exceptions in other processes.

So I guess we should either give a reasonable default, or compute it based on the system configuration. Like `Process.getrlimit(Process::RLIMIT_NOFILE).first / 16` or `8`

What do you think?