def get_active_sessions(self):
pass<EOL>
Returns all sessions in the EIS that are considered active, meaning all
sessions that haven't been stopped/expired.  This is primarily used to
validate potential orphans.

If there are no active sessions in the EIS, this method may return an
empty collection or None.

Performance
-----------
This method should be as efficient as possible, especially in larger
systems where there might be thousands of active sessions.  Large
scale/high performance implementations will often return a subset of
the total active sessions and perform validation a little more
frequently, rather than return a massive set and validate infrequently.
If efficient and possible, it would make sense to return the oldest
unstopped sessions available, ordered by last_access_time.

Smart Results
-------------
*Ideally*, this method would only return active sessions that the EIS
was certain should be invalided.  Typically that is any session that is
not stopped and where its last_access_timestamp is older than either
session timeout (idle or absolute).

For example, if sessions were backed by a relational database or SQL-92
'query-able' enterprise cache, you might return something similar to
the results returned by this query (assuming SimpleSession(s) were
being stored):

    SELECT *
    FROM sessions s
    WHERE s.lastAccessTimestamp < {idle_timeout} and
          s.lastAccessTimestamp < {absolute_timeout} and
          s.stopTimestamp is null

:returns: a Collection of session(s) that are considered active, or an
          empty collection or None if there are no active sessions