Fixes https://github.com/spring-projects/spring-integration/issues/4008 Impossible to retrieve current leader id from ZK `LeaderInitiator` * change return type of `LeaderInitiator.getContext()` from `Context` to `CuratorContext` * add `getLeader()` method to `CuratorContext` * add `getParticipants()` method to `CuratorContext`