Fixed bad bug related to AcornBackupProvider

Fixed bad bug related to AcornBackupProvider

AcornBackupProvider was invoking GraphClientImpl2.makeSnapshot with true
parameter which caused the invocation to write the main.state file when
backups were made which was totally unintentional. The main.state file
is now only written only when the database session is shut down, i.e.
not at all in ClusterManager.makeSnapshot.

Also includes robustness fixes for Acorn database directory locking by
using the Java NIO API for atomic existence testing and creation of the
lock file.

refs #7186

Change-Id: Id668528f8aea789945a4ae3a212d14b07554ba0d