Provide idempotency support to handle duplicate maintenance events

[efeg]

This PR resolves #1347.

[Lincong]

The overall implementation looks good. But I am wondering whether we can consider the following two options.

1. Use caffeine cache (link). Maybe this use case is simple enough and we do not want to introduce a new dependency for it?

2. Create a separate class MaintenanceEventCache that includes the cache cleaner, cache size/ time retention, etc. There could be several advantages for doing it.

• Improve simplicity/readability/modularity of the MaintenanceEventDetector class
• The concurrency control can happen on the MaintenanceEventCache class level instead of MaintenanceEventDetector level (currently, removeOldEventsFromIdempotenceCache and ensureIdempotency methods are synchronized on the MaintenanceEventDetector level). It makes more sense to make MaintenanceEventCache handle its own concurrent data access.
• Improve testability since it's more modular

[efeg]

Thanks for the review.

1. Correct -- adding dependency on something like Caffeine (or Guava) for this simple need would be an overkill.
2. It makes sense to have a separate class -- please see IdempotenceCache.