[performance](load) increase max_broker_concurrency to 100 (#44929)

Increase default `max_broker_concurrency` to 100 to improve broker load
performance.
This option will affect the max number of scan / sink instances allowed
in a broker load.

```
parallel instance = min(max_broker_concurrency,
                        source file size / min_bytes_per_broker_scanner,
                        num backends * load_parallelism)
```

S3 load time of tpcds_1000g catalog_sales:
* before: 438s
* after: 225s

Author: kaijchen

fe/fe-common/src/main/java/org/apache/doris/common/Config.java

@@ -784,7 +784,7 @@ public class Config extends ConfigBase {
 
     @ConfField(mutable = true, masterOnly = true, description = {
             "单个 broker scanner 的最大并发数。", "Maximal concurrency of broker scanners."})
-    public static int max_broker_concurrency = 10;
+    public static int max_broker_concurrency = 100;
 
     // TODO(cmy): Disable by default because current checksum logic has some bugs.
     @ConfField(mutable = true, masterOnly = true, description = {