[enhencement](config) change default memtable size & loadStreamPerNode & default load parallelism (#28977)

We change memtable size from 200MB to 100MB to achieve smoother flush
performance. We change loadStreamPerNode from 20 to 60 to avoid stream
rpc to be the bottleneck when enable memtable_on_sink_node. We change
default s3&broker load parallelsim to make the most of CPUs on moderm
multi-core systems.

Signed-off-by: freemandealer <freeman.zhang1992@gmail.com>

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

@@ -1325,7 +1325,7 @@ public class SessionVariable implements Serializable, Writable {
     public boolean enableMemtableOnSinkNode = true;
 
     @VariableMgr.VarAttr(name = LOAD_STREAM_PER_NODE)
-    public int loadStreamPerNode = 20;
+    public int loadStreamPerNode = 60;
 
     @VariableMgr.VarAttr(name = GROUP_COMMIT)
     public String groupCommit = "off_mode";