code: public static String linkColumnName(FieldDefinition linkDef, String objID) {
          assert linkDef.isLinkField();
          StringBuilder buffer = new StringBuilder();
          buffer.append("~");
          buffer.append(linkDef.getName());
          buffer.append("/");
          buffer.append(objID);
          return buffer.toString();
      }
docs: Return the column name used to store the given value for the given link. The column
  name uses the format:
  <pre>
  ~{link name}/{object ID}
  </pre>
  This method should be used for unsharded links or link values to that refer to an
  object whose shard number is 0.
  
  @param linkDef   {@link FieldDefinition} of a link.
  @param objID     ID of an object referenced by the link.
  @return          Column name that is used to store a value for the given link and
  object ID.
  @see             #shardedLinkTermRowKey(FieldDefinition, String, int)