perf(client): Remove parked senders when a checkout is dropped to prevent memory leaks

[klausi]

Attempt to fix #1315 .

Note that I'm not sure what I'm doing here is right because I have not groked yet what the purpose of those parked senders is.

[seanmonstar]

Thanks for getting it this far, it helped me understand where the leak is. Completely removing the parked list could hurt any other pending requests, making the client use idle connections less often. I've opened #1325 that will cleanup when a Checkout is dropped, but only if the Sender is dead.

[klausi]

Very good, glad I could help at least some way :-)