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
Considering suggesting `Process.getrlimit(Process::RLIMIT_NOFILE).first` instead. That'd still EMFILE though, so it's not really a better default.