Client: chunked upload: Better performance for network stream inputs.

PHP network streams return ~8k per fread() call, even if you asked for
more.  The chunked upload code would ask for 4MB, but then upload
whatever it got back, which yielded a Dropbox API round-trip per ~8k.

Change chunked uploader to read the full 4MB from the stream before
making the API call.

Thanks to Daniel Fenaux and Sergey Morozov.