[SPARK-4013] Do not create multiple actor systems on each executor

In the existing code, each coarse-grained executor has two concurrently running actor systems. This causes many more error messages to be logged than necessary when the executor is lost or killed because we receive a disassociation event for each of these actor systems.

This is blocking #2840.

Author: Andrew Or <andrewor14@gmail.com>

Closes #2863 from andrewor14/executor-actor-system and squashes the following commits:

44ce2e0 [Andrew Or] Avoid starting two actor systems on each executor