ObjectCacher: wait for all reads when stopping flusher

Stopping the flusher is essentially the shutdown step for the
ObjectCacher - the next thing is actually destroying it.

If we leave any reads outstanding, when they complete they will
attempt to use the now-destroyed ObjectCacher. This is particularly a
problem with rbd images, since an -ENOENT can instantly complete many
readers, so the upper layers don't wait for the other rados-level
reads of that object to finish before trying to shutdown the cache.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>