Events and notifications stay disabled until restart when the remote Solr server cannot be reached during startup

The same problem remains for the search core (SolrIndexAvailableLocalesListener, through SolrClientInstance) and for the extension index (ExtensionIndexStore): followed up in XWIKI-25257, to keep this issue on the event store.

The fix also keeps a first attempt during the initialization (logged as a warning when it fails, without failing the initialization), so that with Solr available the events core is still created, migrated and initialized during the startup. After a failure the events core is not requested again for 10 seconds, the event store logs one warning when the core becomes unavailable and one information message when it is available again.