Use StringBuilder instead of StringBuffer

```java

private StringBuffer currentLine = new StringBuffer();

....

public String nextToken() {
while (st.hasMoreTokens()) {
String t = st.nextToken();
if (t.equals("\n")) {
++ line;
currentLine.setLength(0);
} else {
currentLine.append(t);
}
if (!isWhitespace(t)) {
return t;
}
}
throw new IllegalArgumentException("unexpected end of schema");
}
```

Use `StringBuilder` instead of `StringBuffer` as `StringBuffer` is synchronized (which is not required here).

**Reporter**: [David Mollitor](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=belugabehr) / @belugabehr

_{**Note**: *This issue was originally created as [PARQUET-1921](https://issues.apache.org/jira/browse/PARQUET-1921). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}