void erase() {
        map = null;
        next.prev = prev;
        prev.next = next;
        prev = this;
        next = this;
    }
A snapshot of some timeline data structures.
<br>
The snapshot contains historical data for several timeline data structures.
We use an IdentityHashMap to store this data.  This way, we can easily drop all of
the snapshot data.
server-common/src/main/java/org/apache/kafka/timeline/Snapshot.java