Merge branch 'jk/lsan-race-with-barrier'

CI jobs that run threaded programs under LSan has been giving false
positives from time to time, which has been worked around.

* jk/lsan-race-with-barrier:
  grep: work around LSan threading race with barrier
  index-pack: work around LSan threading race with barrier
  thread-utils: introduce optional barrier type
  Revert "index-pack: spawn threads atomically"
  test-lib: use individual lsan dir for --stress runs