Make number of upload threads configurable

I have the following situation: 15 GB of files, lots of them over 20 MB and a very slow (less than 1Mbps) upload bandwith
I try to sync to OneDrive

I observe the following: Insync starts 12 upload threads and every few seconds one of the upload bars goes up by 512KB.
So with files over 20 MB this obviously takes a long time.
Large files often upload about 10 MB and then abort (with an error that looks like a timeout error to me; I also created a thread about the exact error here: Error: "The cloud is temporarily unavailable or under maintenance" when uploading large files ).

I assume this happens because the upload bandwith is split up among the 12 threads - if one upload thread had all the bandwith to itself it surely would be able to upload large files.

So my feature request is to be able to set the number of upload threads the program uses.

Thank you so much for your insight, @ctonic! Let me bring this up with our engineers so we can check how to further improve the sync performance, especially if handling a big sync job.