Add an event uuid to schema and support to write it

[stevehodgkiss]

This will be used to uniquely identify events when mapping new streams, and also to implement idempotent writes in the future.

[stevehodgkiss]

This is a breaking change. In order to update to this version a uuid column needs adding to events with this sql:

CREATE EXTENSION IF NOT EXISTS "uuid-ossp";
ALTER TABLE events ADD COLUMN uuid uuid default uuid_generate_v4() not null;

@mjward @orien @macosgrove @createdbypete