[SPARK-19757][CORE] DriverEndpoint#makeOffers race against...

[SPARK-19757][CORE] DriverEndpoint#makeOffers race against CoarseGrainedSchedulerBackend#killExecutors

## What changes were proposed in this pull request?
While some executors are being killed due to idleness, if some new tasks come in, driver could assign them to some executors are being killed. These tasks will fail later when the executors are lost. This patch is to make sure CoarseGrainedSchedulerBackend#killExecutors and DriverEndpoint#makeOffers are properly synchronized.

## How was this patch tested?
manual tests

Author: Jimmy Xiang <jxiang@apache.org>

Closes #17091 from jxiang/spark-19757.