File descriptor leaks on design document update and view cleanup

If we add a design document with views defined in it, open the corresponding view group (by querying one of its views for e.g.), then update the design document in such a way that the view signature changes (changing a view's map function code for e.g), the old view group remains open forever (unless a server restart happens) and keeps it's view file reference counter active forever.
If a view cleanup request comes, the old view file is not deleted since the reference counter is not dropped by the old view group, keeping the file descriptor in use forever.

This leakage is different from what is reported in COUCHDB-1129 but it's somehow related.

The attached patch, simply shutdowns a view group when the design document is updated and the new view signature changes, releasing the old view file descriptor (as soon as no more clients are using the old view).