ENG-5608: Only turn on the task log when the stream snapshot fragment arrives at
the site during join.

The task log was turned on all the time for join, which logs all fragments it
sees. It is possible that it could see a partial MP transaction. It causes NPE
when we replay the partial transaction and try to log to DR.
ENG-5608: Refactor *joinProducer to address Izzy's review comments.

Pull up the snapshot completion monitor and specicify the correct logger.
ENG-5608: Remove local snapshot completion monitor from RejoinProducer since
it's been pulled up to the base class.
Merge pull request #1264 from VoltDB/ENG-5608

ENG-5608: Only turn on the task log when the stream snapshot fragment arrives at the site during join

r=izzy