Enhance SingleSignOnAuthenticationMechanism implementation to further simplify clustering integration.
 * Extract SingleSignOnManager interface - make AbstractSingleSignOnAuthenticationMechanism concrete
 * Add InMemory implementation of SingleSignOnManager, based on previous SingleSignOnAuthenticationMechanism implementation.
 * Make SingleSignOnEntry an interface, move implementation to InMemorySingleSignOnManager
 * Move SingleSignOn identifier generation responsibility to SSO manager, clustered SSO will want to override this
 * Add SessionManager.getDeploymentName() to uniquely identify session manager, add hashCode(), equals(...) methods
 * Now that SessionManager is hashable, map sessions by session manager instead of session id - session ids are not necessarily unique across deployments.
   - Fetching sessions via session manager (instead of deployment name) simplifies the clustering code, which will need to lookup the Session from the SessionManager via its session id
Other SingleSignOnAuthenticationMechanism fixes:
 * SSO id session attribute is accessed during sessionDestroyed listener but is never set.  Set the attribute during session assocation.
 * Set of seenSessionManagers is read but never populated
 * Use WeakHashMap to fix memory leak of session manager instances in seenSessionManagers set following undeploy