Persists the notifications in the database after delivery flagging them as delivered
This runs a thread to check for the expiry of the delivered notifications and remove them from the database (default is checking every day and removing delivered notifications older than 30 days)
The same interface (CleanupService.java) can be implemented again in future work for removing notifications and sending to kafka.
Closes #16
CleanupService.java
) can be implemented again in future work for removing notifications and sending to kafka. Closes #16