ENG-4895 Add new test for index availability and did various merges.
ENG-4895 Fix CentOS compile issue.
ENG-4941 Throttle elastic index creation.
ENG-4895 Fix stream activation error with no tuples.
Merge commit 'c661b6e' into ENG-4941
ENG-4895 Respond to review comments and rework stream purging.
ENG-4895 Respond to review comments and rework stream purging - part deux.
ENG-4895 Respond to review comments and rework stream purging - part trois.
Merge remote-tracking branch 'origin/ENG-4941' into ENG-4895
ENG-4895/ENG-4941 More review changes and merge ENG-4941.
Merge pull request #631 from VoltDB/ENG-4895

ENG-4895 Elastic index doesn't block snapshots