[tx] Handling retracting schema characteristics, like `:db/cardinality` and `:db/unique`.

[ncalexan]

After #370 (fixing #294 and #295), we still need to implement handling [:db/retract :attribute :db/* ...] of schema characteristics. This might also address [:db/retractEntity :attribute] if it comes after #378.

The tricky thing to ensure here is that we fall back to the default correctly, and that the "schema" materialized view does not get out of sync with the in-memory state.