[SPARK-4750] Dynamic allocation - synchronize kills

Simple omission on my part.

Author: Andrew Or <andrew@databricks.com>

Closes #3612 from andrewor14/dynamic-allocation-synchronization and squashes the following commits:

1f03b60 [Andrew Or] Synchronize kills

(cherry picked from commit 65f929d5b3a50a73cd6397bd4b72c3e7d94c99d7)
Signed-off-by: Josh Rosen <joshrosen@databricks.com>