When AddByPath() is called concurrently on 2 different nodes,
internal path components may be created twice. This violates some
of our assumptions in GetByPath() and, indirectly, in S3 handling of
GetSubTree() results.
Add a test for the correct behaviour, fixes will follow.
Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>
