ObjectCacher: fix debug log level in split

Level 0 should never be used for this kind of debugging.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>
(cherry picked from commit cb3ee33532fb60665f39f6ccb1d69d67279fd5e1)