[SPARK-20955][CORE] Intern "executorId" to reduce the memory usage

## What changes were proposed in this pull request?

In [this line](https://github.com/apache/spark/blob/f7cf2096fdecb8edab61c8973c07c6fc877ee32d/core/src/main/scala/org/apache/spark/scheduler/cluster/CoarseGrainedSchedulerBackend.scala#L128), it uses the `executorId` string received from executors and finally it will go into `TaskUIData`. As deserializing the `executorId` string will always create a new instance, we have a lot of duplicated string instances.

This PR does a String interning for TaskUIData to reduce the memory usage.

## How was this patch tested?

Manually test using `bin/spark-shell --master local-cluster[6,1,1024]`. Test codes:
```
for (_ <- 1 to 10) { sc.makeRDD(1 to 1000, 1000).count() }
Thread.sleep(2000)
val l = sc.getClass.getMethod("jobProgressListener").invoke(sc).asInstanceOf[org.apache.spark.ui.jobs.JobProgressListener]
org.apache.spark.util.SizeEstimator.estimate(l.stageIdToData)
```
This PR reduces the size of `stageIdToData` from 3487280 to 3009744 (86.3%) in the above case.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #18177 from zsxwing/SPARK-20955.

core/src/main/scala/org/apache/spark/ui/jobs/UIData.scala

@@ -20,6 +20,8 @@ package org.apache.spark.ui.jobs
 import scala.collection.mutable
 import scala.collection.mutable.{HashMap, LinkedHashMap}
 
+import com.google.common.collect.Interners
+
 import org.apache.spark.JobExecutionStatus
 import org.apache.spark.executor._
 import org.apache.spark.scheduler.{AccumulableInfo, TaskInfo}
@@ -141,6 +143,14 @@ private[spark] object UIData {
   }
 
   object TaskUIData {
+
+    private val stringInterner = Interners.newWeakInterner[String]()
+
+    /** String interning to reduce the memory usage. */
+    private def weakIntern(s: String): String = {
+      stringInterner.intern(s)
+    }
+
     def apply(taskInfo: TaskInfo): TaskUIData = {
       new TaskUIData(dropInternalAndSQLAccumulables(taskInfo))
     }
@@ -155,8 +165,8 @@ private[spark] object UIData {
         index = taskInfo.index,
         attemptNumber = taskInfo.attemptNumber,
         launchTime = taskInfo.launchTime,
-        executorId = taskInfo.executorId,
-        host = taskInfo.host,
+        executorId = weakIntern(taskInfo.executorId),
+        host = weakIntern(taskInfo.host),
         taskLocality = taskInfo.taskLocality,
         speculative = taskInfo.speculative
       )