[tx] Handle :db.install/attribute in transactor

[ncalexan]

Right now, the transactor doesn't allow to update (or modify) the schema. After #283, this'll be ready for action. This ticket tracks:

• recognizing and acting on :db.install/attribute assertions;
• updating the relevant metadata structures;
• updating the idents and schema SQL materialized views;
• testing.

[ncalexan]

I'll be working on this next.