Probably a good idea to synchronize access to a Singleton.
Introduce a new schema - #55: Change scheme of workitemcomments of old
data
https://github.com/oerich/ReqtDisc/issues/issue/55
When the datasource changes, close all connections. Otherwise, the
DAOManager still operates on the previous database.
Change interface of DiscussionEventDAO #55: Change scheme of
workitemcomments of old data 
https://github.com/oerich/ReqtDisc/issues/issue/55
Refactoring: rename method, setWorkitemcommentid -->
setDiscussionEventID
fixed #55