The ExecutorAllocationManager should not adjust the target number of
executors when killing idle executors, as it has already adjusted the
target number down based on the task backlog.

The name `replace` was misleading with DynamicAllocation on, as the target number
of executors is changed outside of the call to `killExecutors`, so I adjusted that name.  Also separated out the logic of `countFailures` as you don't always want that tied to `replace`.

While I was there I made two changes that weren't directly related to this:
1) Fixed `countFailures` in a couple cases where it was getting an incorrect value since it used to be tied to `replace`, eg. when killing executors on a blacklisted node.
2) hard error if you call `sc.killExecutors` with dynamic allocation on, since that's another way the ExecutorAllocationManager and the CoarseGrainedSchedulerBackend would get out of sync.

Added a unit test case which verifies that the calls to ExecutorAllocationClient do not adjust the number of executors.

Author: Imran Rashid <irashid@cloudera.com>

Closes #20604 from squito/SPARK-23365.