All CI tables should use CI database connection

Today we are only using the CI connection for the ci_instance_variables table. This is because we originally had a rollout plan that involved doing this table first and we haven't yet got around to using the CI connection for all the CI tables (including tags/taggings).

This change will have the downside that running locally decomposed will only work if you share write connection as in #341451 (closed) until we solve all problems relating to write connection.

But it will have the advantage that local development progress will map to the overall rollout plan in &6160 (closed)