For ENG-6153, try handling durabillity notifications in the site threads
Merge pull request #1326 from VoltDB/ENG-6153-2

For ENG-6153, try handling durabillity notifications in the site threads...
Merge branch 'master' of github.com:VoltDB/voltdb into ENG-6075-truncate
Only NULL the previous truncate table pointer when it is not NULL