Move temporary importer data into a separate database

[mgrbyte]

The excise-tmp-data step is causing issues in the migration process - it never completes (in any timely fashion) due to the nature/misuse of datomic's excise API.

https://groups.google.com/forum/#!topic/datomic/0hN6cFZgqBo

The impoter namespace only contains two attributes:
temp - An identifier used as scaffolding by the timestamp-aware importer
ts-name - Username from legacy ACeDB timestamp

• Create a temporary database that persists just for the life-time of the import process.
  (Deleting the database at the end of a migration run)
• Remove the command from the CLI, and the corresponding step in the db-migration pipeline code.