WIP: add cookie_id to primary key, to make event_id unique per session

[borft]

Follow-up of https://docs.google.com/document/d/1aGXpEXfaf4-oFrOgis64Zwaqt2weZrUkyybJJX0WPy8/edit

To migrate an existing db:

alter table data drop constraint data_pkey;
alter table data add primary key (event_id,cookie_id);