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).
If `propertyKeyTranslation` was null (or empty) when no duplicates were found we could optimise the migration of the property store by just doing a straight copy here instead of translating all records to the same as they already are... This was not done in the original `1.9 -> 2.0` migration, so it's not done now either.