Shutdown blocks for 5s if APM Server is down

On shutdown, we're trying to gracefully stop the reporter and the disruptor. As no events can be processed if APM Server is down, the shutdown always takes as long as the configured timeout.

https://github.com/elastic/apm-agent-java/pull/1897 fixes the issue but we don't actually need the soft flush behavior.
Ideally, we should not only check once if the reporter is in a backoff but periodically to guard against cases where APM Server becomes unavailable during shutdown.