Attribute index support for non-special attributes

* Add AttributeIndex class.
* Each attribute does now have an attribute index cookie. The new
attribute index service methods Node::IndexAttribute() and
IndexCookieForAttribute() create+set/retrieve the cookie. The cookie
is actually the attribute index's tree node.
* Add OldNodeAttribute::IndexCookieForAttribute() so the cookie is
available when the node changes.

NodeListener::NodeChanged(): Provide old value

* Add interface OldNodeAttributes an instance of which is passed to
NodeListener::NodeChanged() to provide the old attribute values
(currently only modification time and file size).
* Also extend PackageLinksListener::PackageLinkNodeChanged() with a
OldNodeAttributes parameter.
* Add OldNodeAttributes implementations for PackageLinkSymlink (inner
class OldAttributes) and UnpackingNode (OldUnpackageNodeAttributes).

Attribute index support for non-special attributes

* Add AttributeIndex class.
* Each attribute does now have an attribute index cookie. The new
attribute index service methods Node::IndexAttribute() and
IndexCookieForAttribute() create+set/retrieve the cookie. The cookie
is actually the attribute index's tree node.
* Add OldNodeAttribute::IndexCookieForAttribute() so the cookie is
available when the node changes.

NodeListener::NodeChanged(): Provide old value

* Add interface OldNodeAttributes an instance of which is passed to
NodeListener::NodeChanged() to provide the old attribute values
(currently only modification time and file size).
* Also extend PackageLinksListener::PackageLinkNodeChanged() with a
OldNodeAttributes parameter.
* Add OldNodeAttributes implementations for PackageLinkSymlink (inner
class OldAttributes) and UnpackingNode (OldUnpackageNodeAttributes).