oai-pmh: add oztrack oai-pmh feed service object

- construct OAI-PMH feed record object in OaiPmhRepositoryRecordProducer
- support parent object on records (e.g. pmh feed has repo as parent)
- change dates from String to Date on OaiPmhRecord, serialise in writer
- record datestamp either update or, if null, create date
- check for null before serialising anything in writer; means resulting
  XML may be invalid but is consumer's problem as avoids us crashing