Implement a queue of work for the workers

[dvarrazzo]

This queue would list the table to work on and what work to do on them. The background workers would read from this table to check if there is work to do, using the function(s) defined as per #1.

Observation: this table would have some churn as records are entered modified, removed from there, so it may benefit of a tool to remove bloat from the database. Someone should write it.