[SPARK-17816][CORE] Fix ConcurrentModificationException issue in BlockStatusesAccumulator

## What changes were proposed in this pull request?
Change the BlockStatusesAccumulator to return immutable object when value method is called.

## How was this patch tested?
Existing tests plus I verified this change by running a pipeline which consistently repro this issue.

This is the stack trace for this exception:
`
java.util.ConcurrentModificationException
        at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
        at java.util.ArrayList$Itr.next(ArrayList.java:851)
        at scala.collection.convert.Wrappers$JIteratorWrapper.next(Wrappers.scala:43)
        at scala.collection.Iterator$class.foreach(Iterator.scala:893)
        at scala.collection.AbstractIterator.foreach(Iterator.scala:1336)
        at scala.collection.IterableLike$class.foreach(IterableLike.scala:72)
        at scala.collection.AbstractIterable.foreach(Iterable.scala:54)
        at scala.collection.generic.Growable$class.$plus$plus$eq(Growable.scala:59)
        at scala.collection.mutable.ListBuffer.$plus$plus$eq(ListBuffer.scala:183)
        at scala.collection.mutable.ListBuffer.$plus$plus$eq(ListBuffer.scala:45)
        at scala.collection.TraversableLike$class.to(TraversableLike.scala:590)
        at scala.collection.AbstractTraversable.to(Traversable.scala:104)
        at scala.collection.TraversableOnce$class.toList(TraversableOnce.scala:294)
        at scala.collection.AbstractTraversable.toList(Traversable.scala:104)
        at org.apache.spark.util.JsonProtocol$.accumValueToJson(JsonProtocol.scala:314)
        at org.apache.spark.util.JsonProtocol$$anonfun$accumulableInfoToJson$5.apply(JsonProtocol.scala:291)
        at org.apache.spark.util.JsonProtocol$$anonfun$accumulableInfoToJson$5.apply(JsonProtocol.scala:291)
        at scala.Option.map(Option.scala:146)
        at org.apache.spark.util.JsonProtocol$.accumulableInfoToJson(JsonProtocol.scala:291)
        at org.apache.spark.util.JsonProtocol$$anonfun$taskInfoToJson$12.apply(JsonProtocol.scala:283)
        at org.apache.spark.util.JsonProtocol$$anonfun$taskInfoToJson$12.apply(JsonProtocol.scala:283)
        at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
        at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
        at scala.collection.immutable.List.foreach(List.scala:381)
        at scala.collection.generic.TraversableForwarder$class.foreach(TraversableForwarder.scala:35)
        at scala.collection.mutable.ListBuffer.foreach(ListBuffer.scala:45)
        at scala.collection.TraversableLike$class.map(TraversableLike.scala:234)
        at scala.collection.AbstractTraversable.map(Traversable.scala:104)
        at org.apache.spark.util.JsonProtocol$.taskInfoToJson(JsonProtocol.scala:283)
        at org.apache.spark.util.JsonProtocol$.taskEndToJson(JsonProtocol.scala:145)
        at org.apache.spark.util.JsonProtocol$.sparkEventToJson(JsonProtocol.scala:76)
`

Author: Ergin Seyfe <eseyfe@fb.com>

Closes #15371 from seyfe/race_cond_jsonprotocal.