Fixes alembic migrations for RM6804

* Absorbs 56* revision (subnet_id, address unique constraint) into
  18* (initial revision); oblige already contained the changes in
  56* and prod databases were stamped to 18* so it makes sense to
  absorb contents of 56* into 18*.

* MacAddress.address is already a primary key; removes
  UniqueConstraint on (mac_address_range_id, address) from model.

* Removes added MacAddress unique constraint from 43* revision.
Ah, OK. So, this change seems like the way to go as the unique constraint was redundant to the primary key.