Update extension metadata property keys

As per a discussion with designer folks we will now index extension property keys that are more fool-proof in Clark notation:

key

Any reference to an extension property key will need to be updated with the proper uri. For the time being any lookup that was using a valid old key will use both the old and the new form for backwards compatibility. Also the use of an extension property in the ldap translator should be changed to use the standard format metadata property.

We should also touch on property keys in our developer's guide.