The attached patch fixes the following problems:

in roundup/xmlrpc.py:

* the `close` method commits the database unconditionally.

* For every RoundupRequest instance created, it has to be assured that `close` will be called, even if an unexpected exception occurs in the method. Therefore, all code following the instantiation has to be encapsulated in a try-finally clause. This could be solved easier with a contextmanager, but would require python 2.5...

in test/db_test_base.py:

* when setupTracker finds an already-existing database, it calls `tracker.nuke`, which among other things removes the complete `db` directory, including the `backend_name` file. So after the call to `tracker.nuke`, the database selection has to be repeated.

This patch fixes sf bug #1893892, please review, comment and merge.

Superseded by patch attached to sf bug #1907211