Add option to use syn.store() without exercising multithreads

Thanks !

, I think the release notes (second bullet point) is more useful than linking to this ticket: https://python-docs.synapse.org/build/html/news.html#id2

Kenny pointed me to this fix.

Validated that single threaded upload worked.

tested w/ a 80MB file and saw that the progress updated sequentially in 8MB chunks when synapseclient.config.single_threaded=True and that it updated in larger chunks when synapseclient.config.single_threaded=False depending on the timing of the threads.

I have to note that the performance hit when uploading in single thread mode is huge.

https://github.com/Sage-Bionetworks/synapsePythonClient/pull/574