SessionFactory creates or reuses CDO Session cached in a Map with http
session id as key and a SessionEntry as value.

SessionEntry holds the CDO Session and a timestamp with last http
activity.

SessionManager as own Thread which observes the Map with session
according the http session timeout. Removes and invalidates all entries
which last activity is longer than http session timeout.
Merge pull request #6 from robertblust/session-manager

SessionFactory creates or reuses CDO Session