Create BBO models

What does this MR do and why?

Creates tables and rails models to support BBO framework.

Tables:

• background_operation_workers
• background_operation_jobs
• background_operation_workers_cell_local
• background_operation_jobs_cell_local

NOTE:

• Attributes in background operation worker tables are mostly same as in batched_background_migrations.
  • Currently in BBM next_min_cursor is computed from the jobs table, but it is denormalized and moved within worker table here. So that jobs partitions can be safely dropped once they are completed.
• Attributes in background operation job tables are mostly same as in batched_background_jobs
• Tables are partitioned with a retention period (for non-executables) of 14 days. So that the growth will be contained.

References

• Epic: &16152
• Discussion on table naming and model structure can be found in !203234 (comment 2734378527).
• Discussion on partitioning technique can be found in !199587 (comment 2687529315).
• Discussion on splitting the tables into cell-local and org specific can be found in #562947 (comment 2743808869).
  • The same thread also contains why UUID is chosen, so that it'll be uniq between databases (main, ci) and we don't have explicitly maintain it to make the sequences uniq.

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How to set up and validate locally

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Related to #542944 (closed)