Syncing Event Source on Startup Before Event Processing Starts #788

Consider the following scenario (from the mysql sample):
When there is already a custom resource that represents a schema and the previous reconciliation already created the schema, just the operator crashed after, so neither the status or observed generation were updated. Now: the operator restarts, we check that if the schema is there within the EventSource cache, but the cache was not filled yet since the resource event was processed before. Therefore it tries to create the schema already exists.

To cover such race out of sync situations we should wait until with event processing until the event sources which are managing a cache are filled the cache on startup with the related resources.

In controller runtime there is an analogy called SyncingSource: https://github.com/kubernetes-sigs/controller-runtime/blob/e52a8b1fba620f42bf6be7c6904c77a6da342898/pkg/source/source.go#L64

See usage here: https://github.com/kubernetes-sigs/controller-runtime/blob/e52a8b1fba620f42bf6be7c6904c77a6da342898/pkg/internal/controller/controller.go#L188-L211

So the workflow should be similar in our case, initiate a start on all event sources, wait after all of the is "synced" and start processing events after this happened.

Nota that the syncing is not that simple in some cases, the PerResourcePollingEventSource might want to for all the resources in ControllerResourceEventSource and populate the cache, before it starts.

An other approach (a quick work around?) is to try to get the resources from the suppliers in event sources if not present in the cache, see:
https://github.com/java-operator-sdk/java-operator-sdk/blob/12f6125024d15c9a3963ba4fab2b75687d30d992/operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/event/source/polling/PerResourcePollingEventSource.java#L126