SPARK-CORE [SPARK-3651] Group common CoarseGrainedSchedulerBackend variables together

from [SPARK-3651]
In CoarseGrainedSchedulerBackend, we have:

    private val executorActor = new HashMap[String, ActorRef]
    private val executorAddress = new HashMap[String, Address]
    private val executorHost = new HashMap[String, String]
    private val freeCores = new HashMap[String, Int]
    private val totalCores = new HashMap[String, Int]

We only ever put / remove stuff from these maps together. It would simplify the code if we consolidate these all into one map as we have done in JobProgressListener in https://issues.apache.org/jira/browse/SPARK-2299.
maybe add some javadoc explaining what this is for ...