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.

Crack at implementing: https://github.com/hashicorp/consul/issues/238
Let's be more conservative. Better to switch on `Behavior`. If blank, default to Release, if Release or Delete great, if default (e.g. undefined behavior) return an error.