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 is the change needed to ensure that we test the property upgrade properly:
Bring back the `1.9` version of `propkeydupdb.zip` from the dead (I checked out the old version), and use it instead of the pre-upgraded `2.0` version (that is deleted by this commit). Evident here from changing the relative path through the change from `Legacy20Store.class` to `Legacy19Store.class`.