Attempt to fix leak

[mtryfoss]

This is an approach to fix leaking subscriptions.

I've tested it with about thousand normal calls and it show no leak using pprof yet. I'm going to test a lot more, but I would like your opinions.

Much of the code is pulled from ari/stdbus.

It still doesn't close unused NATS-subscriptions, but I find that not a big issue since they anyway are reused.

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This change is