void unref(final long index) {
        final AtomicInteger refs = getRefs(index);
        if (refs.decrementAndGet() == 0) {
            if (this.refMap.remove(index, refs)) {
                destroySnapshot(getSnapshotPath(index));
            }
        }
    }
Snapshot storage based on local file storage.

@author boyan (boyan@alibaba-inc.com)
<p>
2018-Mar-13 2:11:30 PM
jraft-core/src/main/java/com/alipay/sofa/jraft/storage/snapshot/local/LocalSnapshotStorage.java