Cancelling Java bindings' AsyncUtil.whileTrue

There does not seem to be any downside to replacing `while (true)` in `LoopPartial.apply` with `while (!done.isDone())`.

This is by no means a comprehensive solution for code that composes futures. For example, it does not help `Database.runAsync`, where cancelling only causes the transaction not to be closed.

Start in the Java bindings at AsyncUtil.whileTrue, focusing on LoopPartial.apply and its use of while (true). Confirm that cancellation should stop the loop when done is complete, while noting that Database.runAsync is outside this proposed change. Done means the loop responds to cancellation without claiming to solve broader composed-future cleanup.

Written by the indexing model from the issue text.