Changed the default MQTT handler size to 1/2 number of processors & Fixed some default parameters when core number is 1 (apache#16363)

* fix

* fix

Author: Caideyipi

iotdb-core/datanode/src/main/java/org/apache/iotdb/db/conf/IoTDBConfig.java

@@ -112,7 +112,7 @@ public class IoTDBConfig {
   private int mqttPort = 1883;
 
   /** The handler pool size for handing the mqtt messages. */
-  private int mqttHandlerPoolSize = 1;
+  private int mqttHandlerPoolSize = Math.max(1, Runtime.getRuntime().availableProcessors() >> 1);
 
   /** The mqtt message payload formatter. */
   private String mqttPayloadFormatter = "json";

iotdb-core/datanode/src/main/java/org/apache/iotdb/db/conf/IoTDBDescriptor.java

@@ -528,7 +528,7 @@ public void loadProperties(TrimProperties properties) throws BadNodeUrlException
                 "degree_of_query_parallelism", Integer.toString(conf.getDegreeOfParallelism()))));
 
     if (conf.getDegreeOfParallelism() <= 0) {
-      conf.setDegreeOfParallelism(Runtime.getRuntime().availableProcessors() / 2);
+      conf.setDegreeOfParallelism(Math.max(1, Runtime.getRuntime().availableProcessors() / 2));
     }
 
     conf.setMergeThresholdOfExplainAnalyze(
@@ -2541,7 +2541,7 @@ private void loadCQProps(TrimProperties properties) {
                 "continuous_query_thread_num",
                 Integer.toString(conf.getContinuousQueryThreadNum()))));
     if (conf.getContinuousQueryThreadNum() <= 0) {
-      conf.setContinuousQueryThreadNum(Runtime.getRuntime().availableProcessors() / 2);
+      conf.setContinuousQueryThreadNum(Math.max(1, Runtime.getRuntime().availableProcessors() / 2));
     }
 
     conf.setContinuousQueryMinimumEveryInterval(