Move transfer session FSICs to their own table

Background

The transfer session model tracks both client and server FSICs within JSON database fields for a directional sync. As time has gone on, these fields have begun to represent a significant amount of data. The data within these fields are then used to generate complex query filters against other tables within the database when queuing data for a sync.

Summary

As someone concerned with performance and scaling of Morango, I'd like to move these FSIC fields to a new, dedicated table, thereby reducing the size of the transfer session table (currently 200+GB). I'd like to take advantage of some performance improvements that can be made by leveraging this data directly within the database when queuing records for syncing.

Deliverables

• A new table TransferSessionCounter should be created with fields:
  • transfer_session FK
  • instance_id UUID
  • partition nullable text field
  • server_counter nullable integer
  • client_counter nullable integer
• The table should be have a unique constraint on transfer_session and instance_id
• API endpoints should serialize the table's data into two separate client_fsic and server_fsic fields to maintain compatibility
• Queuing operation queries should join against this new table

Notes

Some progress has been made but was done prior to several recent changes to queuing queries https://github.com/learningequality/morango/commit/9f919c997b79171143e953e9f8d65f5a5c8740da

Start by locating the transfer session model, its JSON FSIC fields, the API serialization, and the queuing operation queries. Review commit 9f919c997b79171143e953e9f8d65f5a5c8740da alongside the recent queuing changes. Done means the new TransferSessionCounter table and constraint exist, API fields remain compatible, and queuing queries join against the table.