[conf](parallel) Reduce parallel tasks for large cluster (apache#38196)

For large cluster, too many parallel tasks will cause performance issue.
So this PR limit the max parallel tasks in Doris.

fe/fe-core/src/main/java/org/apache/doris/qe/SessionVariable.java

@@ -799,7 +799,7 @@ public class SessionVariable implements Serializable, Writable {
     public String timeZone = TimeUtils.getSystemTimeZone().getID();
 
     @VariableMgr.VarAttr(name = PARALLEL_EXCHANGE_INSTANCE_NUM)
-    public int exchangeInstanceParallel = -1;
+    public int exchangeInstanceParallel = 100;
 
     @VariableMgr.VarAttr(name = SQL_SAFE_UPDATES)
     public int sqlSafeUpdates = 0;