Neo4j storage: update index on role retype (#389).

The association metadata index is updated when an *association role* is retyped.

Pending:
    - ... when a  *topic* is retyped.

=> Building composite types works for the first with the new storage layer.

BREAKING CHANGES

The association metadata index format has slightly changed.
Existing DBs *can* be used without modification. However, roles of existing associations can *not* be retyped.
Only roles of new associations can.

Resetting the DB is recommended.

See ticket 389.