[CALCITE-1399] Make the jcommander SerializationConverter public

When this class is private, JCommander fails because it
cannot instantiate the implementation and fails the program.

avatica/standalone-server/src/main/java/org/apache/calcite/avatica/hsqldb/HsqldbServer.java

@@ -113,9 +113,9 @@ public static void main(String[] args) {
   }
 
   /**
-   * Converter from String to Serialization.
+   * Converter from String to Serialization. Must be public for JCommander.
    */
-  private static class SerializationConverter implements IStringConverter<Serialization> {
+  public static class SerializationConverter implements IStringConverter<Serialization> {
     @Override public Serialization convert(String value) {
       return Serialization.valueOf(value.toUpperCase());
     }