Ephemeral Nodes for via Session behavior settings.

Added a "delete" behavior for session invalidation, in addition to
the default "release" behavior. On session invalidation, the sessions
Behavior field is checked and if it is set to "delete", all nodes owned
by the session are deleted. If it is "release", then just the locks
are released as default.
Clean up tests, use switch to default session.Behavior value if unspecified, unrecognized
Set empty Behavior setting into SessionKeysRelease and flag error for unrecognized values
Merge pull request #487 from amalaviy/ephemeral_keys

Ephemeral Nodes for via Session behavior settings.