Reverted the multithreading improvements (Catalina bug 262)
Reverted the multithreading improvements (Catalina bug 262)

See bug report https://github.com/griff/metaz/issues/262

I was able to reproduce these issues on Catalina with the current 1.0.2 build.
It appears Catalina has issues with the multithread changes I made.
In order to expedite a fix for users, this patch reverts the changes made to make
the RemoteData downloads multithreaded (commit d6a42540c45fc96e0c89e476d8ba98603e036c89)

(Note this does not revert the URL cache changes made in the same which work on Catalina without issue)

griff/metaz,griff/metaz,griff/metaz,griff/metaz,griff/metaz