Simplified declaration of THRESHOLD objects

The ACTIVITIES and ENFORCEMENT clauses on the CREATE THRESHOLD statement are now optional, leading to simpler declaration of THRESHOLD objects.

For example, you can now define an ACTIVITYTOTALTIME threshold using a statement like:
CREATE THRESHOLD TH1 FOR DATABASE WHEN ACTIVITYTOTALTIME > 1 MINUTE STOP EXECUTION
rather than
CREATE THRESHOLD TH1 FOR DATABASE ACTIVITIES ENFORCEMENT DATABASE WHEN ACTIVITYTOTALTIME > 1 MINUTE STOP EXECUTION