Add "dynamic" version of `buffer(n)` and `buffer_unordered(n)`

[ronanM]

In the "dynamic" versions of buffer(n: usize) and buffer_unordered(n: usize), the number of buffered futures defined by n: usize could be replaced by AsRef<AtomicUsize>, allowing it to change dynamically over time.

[taiki-e]

Related: #1956