BUG: avoid overtaxing the system when many studies retrieved

Avoid having too many insert processings active at once
by ensuring that each one completes before the next one is
processed (with storescp --exec-sync option).

http://massmail.spl.harvard.edu/public-archives/slicer-users/2013/006324.html

Might also fix #1978.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21978 3bd1e089-480b-0410-8dfb-8563597acbee