bufferoutputstream: add profiling support for write_fn
bufferoutputstream: add profiling support for write_fn

This just lets us see a bit more about what is going on during write
operations as they pass through the streams. This can be useful as the
stream writing is synchronous and performed on the UI thread generally.

GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview