The expire-redef.bro test was sometimes failing due to the second "Run"
message being printed after (should happen before) the "Expired"
message. Fixed by increasing the time interval between events.
Also reduced the number of events raised to make the test finish more
quickly.
The expiration attribute expression is now evaluated for every use. Thus
later adjustments of the value (e.g. by redefining a const) will now
take effect. Values less than 0 will disable expiration.
