@Override
  public T retain() {
    content().retain();
    return (T) this;
  }
An abstract class that implements most of the {@link ByteBufHolder} interface so the subclass of this class
only have to provide necessary implementation of a few methods.
@param <T> The subclass type.
ambry-utils/src/main/java/com/github/ambry/utils/AbstractByteBufHolder.java