Prevent extra "concurrent" Aggregator prepare() calls from one thread.
Prevent extra "concurrent" Aggregator prepare() calls from one thread.

Helps: bgo#652637 - Don't hold locks across async calls

GNOME/folks,pwithnall/folks,renatofilho/folks,renatofilho/folks,renatofilho/folks2,renatofilho/folks2,renatofilho/folks2,GNOME/folks,pwithnall/folks,renatofilho/folks,renatofilho/folks,GNOME/folks,renatofilho/folks2,pwithnall/folks