De-duplicate property keys when upgrading from 1.9 to 2.1

1.9 allows duplicate property keys, and 2.x doesn't. So any upgrade
from 1.9 to 2.x needs to de-duplicate the property keys.
Also adds a check to refuse to start the database if there are
duplicate property keys (or relationship types, or labels).
This was an invalid assumption that the check for duplicates during startup exposed.
Since the name store does not use id=0, it is offset by one.
(There is still a leap of faith here that all names will fit in a single string record)