SVM deallocation should wait for completion of operations on them

[inducer]

(Normal Buffers do that implicitly.)

• For in-order queues, a simple solution would be holding a reference to the queue that was last used to operate on the SVM, and using that to enqueue_release when it goes out of scope.
• For out-of-order queues, we could hold on to a bounded number of events, waiting on the ones that get booted out.
• Should this be mandatory or optional behavior?

[inducer]

#447 is a very similar problem that can probably reuse the solution for this.