18078: Reacquire llfuse lock before populating collection directory

18078: Reacquire llfuse lock before populating collection directory

Updates the test for generating "conflict" files.

I don't have a test case for the reported bug, however this is the
only place I could find where _entries was being modified without the
lock held.  By reacquiring the lock, it should no longer be possible
to read _entries at the same time as it is being updated.

Also clean up a few "from future" bits that are no longer needed
because we're python 3 only.

Arvados-DCO-1.1-Signed-off-by: Peter Amstutz <peter.amstutz@curii.com>