Warn/Crash if close() or destroy() is called with outstanding references

[edenhill]

To help users track down hang-on-termination bugs we could keep track of messages and other objects currently in the application's care that should have been returned to librdkafka prior to calling the destructor.

[nkirsch]

@edenhill we're seeing some intermittent hangs on shutdown and in a different bug report you stated "recent versions has additional debug code to help troubleshoot termination hangs". Can you point me towards that debugging code?

Thanks!

[edenhill]

This commit adds some information why a broker thread is not terminating:
b8bb295

Enable debug with debug=broker.

It would be great if you could provide a reliable reproduction of this hang.

[nkirsch]

Thanks! We see this intermittently at scale, so we don't yet have a consistent way to reproduce. We're also not yet at that commit, but we'll see what we can do when we get there.

[edenhill]

Thank you. librdkafka 0.11.1 was just released and is the release you want to upgrade to when the time comes.