Fix Race Condition in Integration Tests Using Redis SessionEventRegistry

[rwinch]

The integration tests that use SessionEventRegistry assume that there is only a single event type for each session id. This leads to race conditions that result in ClassCastExceptions. For example:

ReactiveRedisIndexedSessionRepositoryITests > findByPrincipalNameWhenExpireKeyEventThenRemovesIndexAndSessionExpiredEvent() FAILED
    java.lang.ClassCastException at ReactiveRedisIndexedSessionRepositoryITests.java:171