Enhance event handling

[snowdrop-bot]

This PR includes:

• add lifecycle hooks to EventSource (Add lifecycle hooks to EventSource operator-framework/java-operator-sdk#368)
• review DefaultEventSourceManager instance variable modifiers
• make DefaultEventSourceManager::registerEventSource final as it is invoked by the constructor
• DefaultEventSourceManager use containsKey instead of null check to determine if an event source is already registerd
• add lifecycle hooks to the operator so i.e. watchers can be properly closed

Fixes operator-framework#368

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operator-framework#407

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$upstream:407$