Looked into this, and dropKey from web actually removes the url,
so git-annex won't try to get content from it.
So, if lockContent were implemented for web, and the web was left as the
only thing containing an object, another repo could at the same time
drop from web and remove its url, leaving no way to get the object.
Add to that, of course, the web is typically set untrusted, and so
implementing lockContent would not then be useful.
Similar reasoning applies to the bittorrent special remote, as well
as the fact that it does not even implement checkKey.
