[config](move-memtable) set LOAD_STREAM_PER_NODE default to 2 (#30830)

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

@@ -1400,7 +1400,7 @@ public class SessionVariable implements Serializable, Writable {
     public boolean enableMemtableOnSinkNode = true;
 
     @VariableMgr.VarAttr(name = LOAD_STREAM_PER_NODE)
-    public int loadStreamPerNode = 20;
+    public int loadStreamPerNode = 2;
 
     @VariableMgr.VarAttr(name = GROUP_COMMIT)
     public String groupCommit = "off_mode";