ObjectCacher: optionally make writex always non-blocking

ObjectCacher: optionally make writex always non-blocking

Add a callback argument to writex, and a finisher to run the
callbacks. Move the check for dirty+tx > max_dirty into a helper that
can be called from a wrapper around the callbacks from writex, or from
the current place in _wait_for_write().

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>