Previously the APICache would keep file descriptors open until the

program exited. This was because of a call to atexit.register that

increased the reference count of the APICache. However, we always

write to the database after every database change, so this is

unnecessary. Removing the call to atexit.register allows the open

file descriptors to be closed when an APICache goes out of scope.

Ran unit tests.
I ran the test.py file attached in report of issue 3751 before and

after the change.  Before the change, the list of open file

descriptors would grow with each call to the doit function. After

the change, the list of open file descriptors is always be the

empty list.