Allow cancelling of FuturePublished event(s) through a cancellation key.

I went ahead and added the functionality to cancel future published events.

To do the delete I just do a straight `DELETE FROM` instead of doing a 100-item purge. The reason for this is that I assume regular purges are considered more of a background process whereas a cancellation should be more immediate, if possible.

I have provided upgrade scripts and stored procedures for MSSQL and Postgres. I've ran the tests on MSSQL, but not on Postgres (although I did check if I could create the tables and stored procedures on PG as well).