OPENDJ-2476 Purge of file-based changelog is very slow and the changelog size is growing

Monitoring shows generating the cookies makes up for a big part of the processing time.
2 factors make this slow:
* using String.format() which ends up always parsing the same format string
* continuously repeating toString()

Here is how this commit fixes these problems:
* used StringBuilder + Long/Integer.toHexString() + custom padding
* Created several toString(StringBuffer) methods and chained them