[SPARK-19620][SQL] Fix incorrect exchange coordinator id in the physical plan

## What changes were proposed in this pull request?
When adaptive execution is enabled, an exchange coordinator is used in the Exchange operators. For Join, the same exchange coordinator is used for its two Exchanges. But the physical plan shows two different coordinator Ids which is confusing.

This PR is to fix the incorrect exchange coordinator id in the physical plan. The coordinator object instead of the `Option[ExchangeCoordinator]` should be used to generate the identity hash code of the same coordinator.

## How was this patch tested?
Before the patch, the physical plan shows two different exchange coordinator id for Join.
```
== Physical Plan ==
*Project [key1#3L, value2#12L]
+- *SortMergeJoin [key1#3L], [key2#11L], Inner
   :- *Sort [key1#3L ASC NULLS FIRST], false, 0
   :  +- Exchange(coordinator id: 1804587700) hashpartitioning(key1#3L, 10), coordinator[target post-shuffle partition size: 67108864]
   :     +- *Project [(id#0L % 500) AS key1#3L]
   :        +- *Filter isnotnull((id#0L % 500))
   :           +- *Range (0, 1000, step=1, splits=Some(10))
   +- *Sort [key2#11L ASC NULLS FIRST], false, 0
      +- Exchange(coordinator id: 793927319) hashpartitioning(key2#11L, 10), coordinator[target post-shuffle partition size: 67108864]
         +- *Project [(id#8L % 500) AS key2#11L, id#8L AS value2#12L]
            +- *Filter isnotnull((id#8L % 500))
               +- *Range (0, 1000, step=1, splits=Some(10))
```
After the patch, two exchange coordinator id are the same.

Author: Carson Wang <[email protected]>

Closes #16952 from carsonwang/FixCoordinatorId.

Author: carsonwang

sql/core/src/main/scala/org/apache/spark/sql/execution/exchange/ShuffleExchange.scala

@@ -46,7 +46,7 @@ case class ShuffleExchange(
   override def nodeName: String = {
     val extraInfo = coordinator match {
       case Some(exchangeCoordinator) =>
-        s"(coordinator id: ${System.identityHashCode(coordinator)})"
+        s"(coordinator id: ${System.identityHashCode(exchangeCoordinator)})"
       case None => ""
     }