Add ability to adjust length of the buffer underlying a memory channel

I was thinking it would be nice in the future for auto-tuning algorithms to adjust queue max-lengths in real-time without having to copy references around. If there are currently no barriers or complex mechanisms to achieve this (ie all the logic for queue size checks and waiters is local to one place and easily synchronized), then it could add versatility for the future.

As for scenarios that would make use of this, you could implement a reverse proxy that could handle both slow and fast connections efficiently (where connection speed isn't first known), or a single queue whose units of work can be predicted to be following an increasing or decreasing trend over time, and a secondary structure could be used to raise or lower the length based on achieving a specific total weighted sum of the queue's units.

Another ("BoundedDynamicQueue" for instance) class could be used that enables setting the length, but if it really is a couple of lines difference, it may serve better being merged into the default implementation.

Locate the memory-channel implementation and inspect the queue-size checks and waiter synchronization first. Compare an adjustable default implementation with a separate BoundedDynamicQueue, then define the resizing API and synchronization behavior. Done means the chosen design is implemented with coverage for changing the queue length safely.