SPARK-3020: Print completed indices rather than tasks in web UI

Author: Patrick Wendell <pwendell@gmail.com>

Closes #1933 from pwendell/speculation and squashes the following commits:

33a3473 [Patrick Wendell] Use OpenHashSet
8ce2ff0 [Patrick Wendell] SPARK-3020: Print completed indices rather than tasks in web UI

(cherry picked from commit 0c7b452904fe6b5a966a66b956369123d8a9dd4b)
Signed-off-by: Reynold Xin <rxin@apache.org>