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>