Remove exact buffer/window with Supplier<source>? #6555

There exist special overloads of the exact-boundary buffer/window operators that takes a supplier of a source. (Example JavaDocs).

This supplier is called over and over once the previous boundary signaled an item or completed.

I'm not sure if this type of boundary was in use at all. It can be emulated with the plain sourced versiong:

source.buffer(Observable.defer(supplier).take(1).repeat())