temporarily disable decrementing conns_in_use_ when it's already <= 0

Former-commit-id: d2c874147a4b0ae529a5b3b44c9186566ac2d174

cpp/conf/log_config.conf

@@ -20,8 +20,8 @@
     TO_STANDARD_OUTPUT      =   false
 ## Error logs
 * ERROR:
-    ENABLED                 =   false
+    ENABLED                 =   true
     FILENAME                =   "/tmp/milvus/logs/milvus-%datetime{%H:%m}-error.log"
 * FATAL:
-    ENABLED                 =   false
+    ENABLED                 =   true
     FILENAME                =   "/tmp/milvus/logs/milvus-%datetime{%H:%m}-fatal.log"
\ No newline at end of file

cpp/conf/server_config.yaml

 server_config:
   address: 0.0.0.0
-  port: 19530                 # the port milvus listen to, default: 19530, range: 1025 ~ 65534
+  port: 19531               # the port milvus listen to, default: 19530, range: 1025 ~ 65534
   gpu_index: 0                # the gpu milvus use, default: 0, range: 0 ~ gpu number - 1
-  mode: single                # milvus deployment type: single, cluster
+  mode: cluster                # milvus deployment type: single, cluster
 
 db_config:
   db_path: /tmp/milvus
   #URI format: dialect://username:password@host:port/database
   #All parts except dialect are optional, but you MUST include the delimiters
-  db_backend_url: mysql://root:1234@:/test
-  index_building_threshold: 1024          #build index file when raw data file size larger than this value, unit: MB
+  #Currently supports mysql or sqlite
+  db_backend_url: mysql://root:1234@:/test # meta database uri
+  index_building_threshold: 1024   # index building trigger threshold, default: 1024, unit: MB
 
 metric_config:
   is_startup: off      # if monitoring start: on, off

cpp/src/db/MySQLConnectionPool.h

@@ -54,10 +54,12 @@ public:
     void release(const mysqlpp::Connection* pc) override {
         mysqlpp::ConnectionPool::release(pc);
 //        ENGINE_LOG_DEBUG << "conns_in_use_ in release: " << conns_in_use_ << std::endl;
-        --conns_in_use_;
-        if (conns_in_use_ < 0) {
+        if (conns_in_use_ <= 0) {
             ENGINE_LOG_WARNING << "MySQLConnetionPool::release: conns_in_use_ is less than zero.  conns_in_use_ = " << conns_in_use_ << std::endl;
         }
+        else {
+            --conns_in_use_;
+        }
     }
 
     void set_max_idle_time(int max_idle) {