neverending AsyncStore.awaitNotEmpty during stop() of AsyncStore

The stop() of AsyncStore might fail to stop all threads.

If I introduce a delay in the Store implementation, after stop() on AsyncStore methods get stuck in awaitNotEmpty(), waiting forever for a second interrupt.

Attaching a testcase which - in my case - is able to deadlock the tests to show the problem in almost all runs. Will clean it up into a a timeout-limiting test suggestions to have it fail consistently are welcome.

I'm attaching also a fix proposal.