WL-601 Support for deleting events through API.

Conflicts:
	kernel-impl/src/main/sql/mssql/sakai_event.sql
WL-601 Delete old session data.

Conflicts:
	kernel-impl/src/main/java/org/sakaiproject/event/impl/UsageSessionServiceAdaptor.java
	kernel-impl/src/main/java/org/sakaiproject/event/impl/UsageSessionServiceSql.java