Eliminate (another) synchronization bottleneck in FlatMapOp
Merge branch 'master' into bizo-2.10

* master:
  Eliminate (another) synchronization bottleneck in FlatMapOp
Fix concurrency bug in FlatMapOp.next() introduced in commit 8b36ff6 + add better test
Merge branch 'master' into bizo-2.10

* master:
  Fix concurrency bug in FlatMapOp.next() introduced in commit 8b36ff6 + add better test