[system-health] Fix file handle leak (#10059)

- Why I did it
swsscommon.ConfigDBConnector does not automatically close connection when the instance is recycled by python. So, it should not create this instance each time calling check_services. It will cause error like Failed to read from file /var/run/hw-management/led/led_status_capability - OSError(24, 'Too many open files')

- How I did it
Only connect DB once in init

- How to verify it
Manual test

src/system-health/health_checker/service_checker.py

@@ -51,6 +51,8 @@ class ServiceChecker(HealthChecker):
 
         self.need_save_cache = False
 
+        self.config_db = None
+
         self.load_critical_process_cache()
 
     def get_expected_running_containers(self, feature_table):
@@ -248,9 +250,10 @@ class ServiceChecker(HealthChecker):
         Args:
             config (config.Config): Health checker configuration.
         """
-        config_db = swsscommon.ConfigDBConnector()
-        config_db.connect()
-        feature_table = config_db.get_table("FEATURE")
+        if not self.config_db:
+            self.config_db = swsscommon.ConfigDBConnector()
+            self.config_db.connect()
+        feature_table = self.config_db.get_table("FEATURE")
         expected_running_containers, self.container_feature_dict = self.get_expected_running_containers(feature_table)
         current_running_containers = self.get_current_running_containers()