CRM-14727 - Move $isActive from SequenceListener to Events

When a case-change listener makes another change to a case, we don't want to
refire all the case-change listeners.  $isActive helps avoid that (infinite)
recursion.

This commit has two important qualities:

 1. The code is simpler. There's a single set() and single unset().
 2. Developers of new case-change listeners don't need to worry
    about recursion problems -- it's handled centrally.