osdc/ObjectCacher: back off less during flush

In cce990efc8f2a58c8d0fa11c234ddf2242b1b856 we added a limit to avoid
holding the lock for too long.  However, if we back off, we currently
wait for a full second, which is probably a bit much--we really just want
to give other threads a chance.

Backport: emperor
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1038 from ceph/wip-objectcacher-backoff

osdc/ObjectCacher: back off less during flush

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>