Mirror URL table's click column

[yong-jie]

Problem

As we are moving forward with features that require bulk database operations, we want to move click count (a frequently updated field) in the URL table.

This is because updating the click count atomically requires a row lock, and it is unlikely that bulk operations will be able to acquire the necessary locks to proceed if a link is being visited constantly.

Part 1 of #984

Solution

Visit our wiki page for in-depth discussion of solution.

Proposed migration approach involves multiple phases.

1. Deploy this PR that will let sequelize create a new url_clicks table.

2. Run the migration script in this PR that will populate the url_clicks table, as well as create the necessary hooks to keep them always in sync.

3. Deploy a separate PR that changes application code to do read/writes to url_clicks table.

4. Run a migration script to remove hooks and url table's click column.

Side effects of PR:

• Original Clicks model renamed to DailyClicks.

Deploy Notes

Deploy code first to let sequelize create the tables before running migration script.

New scripts:

• 20201207_synchronise_url_clicks.sql : Copies click count into url_clicks table, and adds DB hooks to keep it in sync.

[liangyuanruo]

some suggestions but lgtm!