static <V> V get(RefCountingMap.Payload<V> p) {
    return p == null ? null : p.get();
  }
A map of K to V, but does ref counting for added and removed values. The values are not added
directly, but instead requested from the given Supplier if ref count == 0. Each put() call will
increment the ref count, and each remove() will decrement it. The values are removed from the map
iff ref count == 0.
iotdb-core/consensus/src/main/java/org/apache/iotdb/consensus/ratis/metrics/RefCountingMap.java