Fix JsonReader parallel issue

[alexcheng1982]

This PR fixes an issue with parallel stream used in JsonReader.

Problem

I wrote a simple program to test the JsonReader. The JSON content is very simple.

[
  {
    "name": "Alex",
    "email": "[email protected]",
    "jobTitle": "Software Engineer"
  },
  {
    "name": "Bob",
    "email": "[email protected]",
    "jobTitle": "System Admin"
  }
]

The code shown below extracts keys name and jobTitle and puts them into the Document.

public class JsonReaderSample {

  void read() {
    var metadataGenerator = new JsonMetadataGenerator() {
      @Override
      public Map<String, Object> generate(Map<String, Object> jsonMap) {
        return Map.of("email", jsonMap.getOrDefault("email", ""));
      }
    };
    var resource = new FileSystemResource(
        Path.of(".", "data", "json-array.json"));
    var reader = new JsonReader(resource, metadataGenerator, "name",
        "jobTitle");
    var docs = reader.read();
    docs.forEach(System.out::println);
  }

  public static void main(String[] args) {
    new JsonReaderSample().read();
  }
}

When running this simple program, the output may look like below. The text of keys and values from different keys are mingled (jobTitle: name: System AdminBob).

Document{id='a55153c1-09ab-4fc6-aa18-07a6f20e94d6', metadata={[email protected]}, content='name: Alex
jobTitle: Software Engineer
', media=[]}
Document{id='46b27f32-1ea3-4ca9-a005-9c93195bb335', metadata={[email protected]}, content='jobTitle: name: System AdminBob

', media=[]}

The usage of parallelStream caused the invocations of StringBuffer.append for different keys from different threads intertwined when operating on the shared StringBuffer.

Fix

The parallel processing of multiple keys seems unnecessary, since parallel processing is already enabled for documents in the JSON array, and we are dealing with only in-memory data. So I changed to use normal stream and replaced StringBuffer with StringBuilder.

[markpollack]

Yikes! thanks a ton.

merged in c205c7d