core: Consolidate message sending code in GDataService
core: Consolidate message sending code in GDataService

This reduces duplication of the repetitive code needed to send messages in
the authentication and query code paths, and gives a central location to
fix the race conditions in cancellation.

Pecisk/libgdata,Pecisk/libgdata,Pecisk/libgdata-gtasks,pwithnall/libgdata,pwithnall/libgdata,Pecisk/libgdata-gtasks,Distrotech/libgdata,Pecisk/libgdata,GNOME/libgdata,Distrotech/libgdata,Distrotech/libgdata,Pecisk/libgdata-gtasks