Recycle nodes in unbounded channels

[ghost]

In order to reduce the rate of allocation/deallocation in unbounded channels, we should keep some number of removed nodes for quick recycling.

Here's an idea for how we might implement this:

• Each unbounded channel keeps a lock-free stack of removed nodes.

• If a receive operation removes a node, we should push it onto the stack. However, if the stack is larger than the live queue, that means we're wasting a lot of memory. In such cases we should deallocate the removed node as well as deallocate one node from the stack.

• If a send operation needs a new node, it should first try popping one from the stack. And if the stack is empty, just allocate a fresh node.

[ghost]

This is a pretty advanced optimization with questionable benefits. I'm not sure whether it's worth it anymore.