plumb in LiveUpdate (WIP)

Each command that first checks preferred content (and/or required
content) and then does something that can change the sizes of
repositories needs to call prepareLiveUpdate, and plumb it through the
preferred content check and the location log update.

So far, only Command.Drop is done. Many other commands that don't need
to do this have been updated to keep working.

There may be some calls to NoLiveUpdate in places where that should be
done. All will need to be double checked.

Not currently in a compilable state.

Command/DropKey.hs

@@ -55,5 +55,5 @@ perform key = ifM (inAnnex key)
 
 cleanup :: Key -> CommandCleanup
 cleanup key = do
-	logStatus key InfoMissing
+	logStatus NoLiveUpdate key InfoMissing
 	return True