[CB-5790] Fixed iOS upload loading the whole content of file to memory before uploading.

I'm quite new to iOS development but i believe this works fine since i was  able to repeatedly upload a 260MB file without any memory warning.

Not so sure about error handling though, maybe there's something to do on this side.

Might need to update the doc too since chunked mode doesn't change anything.
I'm still not so sure what it was supposed to be doing, or more exactly why anyone would not want chunked mode.
Anyway, it didn't do much before but now the option is just ignored.

Also it will probably need some cleanup since i commented out a bunch of old code.
I'll do it when the code is somewhat approved :-)

Oh and i almost forgot, i didn't test the <code>ALAssetRepresentation</code> path so i can't be 100% sure it works.