Warn if materializer is not referenced any more but StreamSupervisor is still alive

[jrudolph]

In "real", long-running apps, dropping the reference to the materializer is often a sign that you create too many one-off materializers without shutting them down again after use (potential memory leak).

I guess the StreamSupervisor could monitor a reference queue to see when this happens but it's hard to say if it will introduce spurious warnings.

[jrudolph]

We could also automatically shut down the stream supervisor after the materializer went out of scope and all running streams were finished.

[richardimaoka]

Wanted to see if I can work on this but not sure what to do for

StreamSupervisor could monitor a reference queue

and how to detect

the materializer went out of scope

Much appreciated if you could provide more pointers on this.

[patriknw]

Weak references and such can have a negative impact on GC performance. I'm not sure if that is also true for a reference queue?