core: Tidy up critical regions in GDataUploadStream
core: Tidy up critical regions in GDataUploadStream

Tidy up the mutex locks and unlocks, and ensure that all variables which
should be protected by a mutex, are.

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