[BUG] stats: don't call buffer_shutw(), but ->shutw() instead

[BUG] stats: don't call buffer_shutw(), but ->shutw() instead

Calling buffer_shutw() marks the buffer as closed but if it was already
closed in the other direction, the stream interface is not marked as
closed, causing infinite loops.

We took this opportunity to completely remove buffer_shutw() and buffer_shutr()
which have no reason to be used at all and which will always cause trouble
when directly called. The stats occurrence was the last one.