Add identifier to events

[brinxmat]

In order to allow traceability, I have added an identifier field to events.

Notes:

• The old event constructors have been updated to add a random UUID as a temporary measure, I am not sure that we want to enforce a pattern where each event automatically has a unique identifier since we may wish to trace events from different handlers using the same identifier, i.e. we may wish to pass the same identifier to different events
• The old constructors are marked as @deprecated to facilitate the identification of these methods while we refactor code that uses these implementations