JIRA: https://jira.spring.io/browse/INT-4220
Currently when an error is thrown from the event publishing the role granting is broken and we just go to the role revoking.
* Since it's just an event publishing it shouldn't effect the original leader election.
* `try...catch` event publishing in the `LeaderInitiator` and `logger.warn` an `Exception`
* Make `leader/Context` as `@FunctionalInterface` for simple Lambda use-case like `NULL_CONTEXT` - `() -> false`
* Remove all the internal `NullContext` implementations in favor of above mention Lambda
* In the `LockRegistryLeaderInitiator` use `CustomizableThreadFactory` instead of custom `ThreadFactory` for prefixing
* Add `zookeeper/leader/LeaderInitiator#getContext()` for external usage and consistency with other similar components
* Fix `CuratorContext.toString()` typo
9616cc72a1