Shutdown blocks for 5s if APM Server is down

[felixbarny]

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.

#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.