CalDAV fixes/improvements
CalDAV fixes/improvements

 - store CTag at the fully finished sync, not before
 - listen for ESource changes
 - less aggressive locking
 - sync can be interrupted
 - create/modify/remove operations have precedence before the sync

tintou/evolution-data-server,Distrotech/evolution-data-server,matzipan/evolution-data-server,matzipan/evolution-data-server,gcampax/evolution-data-server,Distrotech/evolution-data-server,tintou/evolution-data-server,gcampax/evolution-data-server,matzipan/evolution-data-server,matzipan/evolution-data-server,Distrotech/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server,gcampax/evolution-data-server,tintou/evolution-data-server,Distrotech/evolution-data-server,gcampax/evolution-data-server,Distrotech/evolution-data-server