ENG-6358: Draining on disk generation and catalog with same genration collide on ack and stops export.
ENG-6358: make sure generation is closed and deleted.
Merge branch 'master' into ENG-6358
ENG-6358: review changes.
Merge branch 'master' into ENG-6358
Merge branch 'master' into ENG-6358
ENG-6358: check existing generation
Merge pull request #1393 from VoltDB/ENG-6358

Eng 6358