deleting DUChain cache dir together with session dir

[David Nolden]

The same thing is true for session-directories, we don't yet delete
all those anonymous temporary sessions.

What we need is some "housekeeping", something like:
* After shutdown, check all existing sessions. If they are anonymous
(eg. not accessible through the menu), not currently open, and not
accessed for more than 48 hours, delete the directory. (We have to be
careful not to delete sessions which are still open somewhere,
consider for example a shared-networking environment)
* Check all existing duchain-directories, and delete all dirs where
the corresponding session-dir doesn't exist.

Greetings, David