[MOD-2328] iOS: Refactor Ti.URLSession, allow background uploads

This is an improvements I was working on a few months ago. It allows people to perform background uploads (together with the already existing background download functionality). While working on it, I also deprecated the old API to have a unifies URLSession api that does it's operations on "Session" objects instead of having all methods on the module top level. I also exposed all other possible properties and methods around it, so people can use all the sugar from the NSURLSession API in Titanium. Thankfully, [~rlustemberg] also did a PR to the core to receive the upload responses from the uploads in [here](https://github.com/appcelerator/titanium_mobile/pull/8860). Together with my module PR [here](https://github.com/appcelerator-modules/ti.urlsession/pull/9), the functionality is completely implemented by now and can be reviewed.