Fix failure when system properties modified when loading configuration

System properties are global mutable state.  Prevent
`ConfigurationLoader` from throwing (`NullPointerException`) when system
properties are modified during `getSystemProperties()`.

CHANGES

@@ -1,3 +1,7 @@
+249
+
+-  Fix failure when system properties modified when loading configuration
+
 248
 
 - Update Jetty to 12.0.9

configuration/src/main/java/io/airlift/configuration/ConfigurationLoader.java

@@ -67,6 +67,9 @@ public static Map<String, String> loadPropertiesFrom(String path)
 
     public static Map<String, String> getSystemProperties()
     {
-        return fromProperties(System.getProperties());
+        Properties systemProperties = System.getProperties();
+        synchronized (systemProperties) {
+            return fromProperties(systemProperties);
+        }
     }
 }