int getLength() {
      return endIndex - startIndex;
    }
A Map that keeps a sorted array in order to provide the concurrent map interface. Keeping a
sorted array means that it's much more cache line friendly, making reads faster than the tree
version. In order to make concurrent reads and writes safe this does a copy on write. There can
only be one concurrent write at a time.
hbase-common/src/main/java/org/apache/hadoop/hbase/types/CopyOnWriteArrayMap.java