Do we want to add writer.waitForDesiredSize() in addition to writer.ready?

[domenic]

In #488 we are keeping the ready promise as a simple writer.ready.

In #318 we talked about more complicated methods, like .waitForDesiredSize(n). (There were lots of options.) After some discussion in person I think we would prefer to just have a single simple promise for now; the need for allowing arbitrary waits is not clear.

However, there is an option of switching from .ready to .waitForDesiredSize() (no arguments), which would in the future allow new abilities by adding arguments.

If we don't make this switch, and in the future want to add more precise options, then the worst thing that happens is that we have two somewhat-redundant/confusing ways of checking the backpressure status: .ready vs. .waitForDesiredSize(). That is not a big deal. So maybe it is fine.

[domenic]

After living with .ready for a few months, I think we can conclude that it's pretty good as-is. If we see a need for .waitForDesiredSize(n) later, we can add that as an addition. So, let me rename this issue.