Submitting large batch of jobs fails on slow network

Default batch size is 20 problems. Submitting 20 full-size problems to an Advantage system/solver amounts to a single SAPI (POST) request with a 9MB payload. That's almost double the part size in the multipart upload scheme we use. Also, on a typical ADSL line, that's over a minute of upload time.

It has been observed such a slow upload request can fail with:
```
OSError: ('Connection aborted.', RemoteDisconnected('Remote end closed connection without response',))
```