CRM-14727 - SequenceListener fixes

1. Fire the case-change event outside the main transaction. When case-change
listeners query the DB, they'll more understandable results.

2. Determine list of case-activities using $case['activities'] rather than
trying to call Activity.get(case_id=X).  Activity.get does not support
filtering on case_id.
CRM-14727 - SequenceListener - Tweak test