wish: make it friendly share an existing database

[markstos]

I may want to run this in a database with other tables. The use-case is having one database per customer, and wanting to have one job queue per customer, since they each have their own Node apps with different configurations.

A couple updates would be helpful:

• Document which tables are used, so it clear if there's going to a conflict.
• Rename the "job_id_idx" index, which is generic enough that it might conflict with a pre-existing index. The name "job_snapshots_job_id_idx" should be sufficiently unique, as should the table name "job_snapshots".

[timlesallen]

Fair comments @markstos. I don't have time to do these changes currently, but I would definitely welcome/consider PRs. :)