For ENG-3569, first stab at backpressuring async command logging
For ENG-3569, tweak comments
Merge pull request #1123 from VoltDB/ENG-3569

For ENG-3569, first stab at backpressuring async command logging
V4.1
Merge branch 'master' of github.com:VoltDB/voltdb into ENG-5712
ENG-5712 Fix unit test.