Add uniqueness and cardinality

[loganmhb]

Right now, all attributes have a cardinality of many and do not enforce uniqueness. These should both be configurable.

For cardinality, "many" returns all non-retracted facts; "one" returns only the most recent non-retracted fact.

For uniqueness, there should be three types:

1. Not unique (the default, current behavior)
2. Unique (transactions creating an entity with a duplicate value fail)
3. Identity (transactions creating an entity with a duplicate value are converted to updates)