#47 only purge completed sessions

[Joe Romaine]

When the [ 616344 ] feature request (i.e. auditing
deletion) was implemented, it was in a way that was
ignorant of whether sessions were active or expired.
This task should only delete entries for expired sessions.
This will require a session expiration listener which
updates a new column on the statistics table. It will
also require a modification to the ImageServlet to
default entries to have an active session. Note that this
enhancement will allow for tracking of who is currently
using the picture server also, but features related to this
should be logged separately.