V remove(K k) {
    Payload<V> p = map.computeIfPresent(k, (k1, v) -> --v.refCount <= 0 ? null : v);
    return p == null ? null : p.v;
  }
Decrements the ref count of k, and removes from map if ref count == 0.
@param k the key to remove
@return the value associated with the specified key or null if key is removed from map.
hbase-metrics/src/main/java/org/apache/hadoop/hbase/metrics/impl/RefCountingMap.java