improve BackgroundLeaseUpdate warning messages

*WARN* [DocumentNodeStore lease update thread (1)] org.apache.jackrabbit.oak.plugins.document.DocumentNodeStore BackgroundLeaseUpdate.execute: time since last renewClusterIdLease() call longer than expected: 5338ms

        @Override
        protected void execute(@Nonnull DocumentNodeStore nodeStore) {
            // OAK-4859 : keep track of invocation time of renewClusterIdLease
            // and warn if time since last call is longer than 5sec
            final long now = System.currentTimeMillis();
            if (lastRenewClusterIdLeaseCall <= 0) {
                lastRenewClusterIdLeaseCall = now;
            } else {
                final long diff = now - lastRenewClusterIdLeaseCall;
                if (diff > 5000) {
                    LOG.warn("BackgroundLeaseUpdate.execute: time since last renewClusterIdLease() call longer than expected: {}ms", diff);
                }
                lastRenewClusterIdLeaseCall = now;
            }
            // first renew the clusterId lease
            nodeStore.renewClusterIdLease();
        }