[BRANCH-1.1][SPARK-4626] Kill a task only if the executorId is (still) registered with the scheduler

v1.1 backport for #3483

Author: roxchkplusony <roxchkplusony@gmail.com>

Closes #3503 from roxchkplusony/bugfix/4626-1.1 and squashes the following commits:

234d350 [roxchkplusony] [SPARK-4626] Kill a task only if the executorId is (still) registered with the scheduler