migrate SerialTaskQueue from tbb::task to tbb::task_group API

12 jobs for switch-to-tbb-taskgroup in 28 minutes and 35 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Build
passed #8475345
cvmfs
build:gcc8:opt

00:11:59

passed #8475347
cvmfs
build:gcc9:dbg

00:18:57

passed #8475346
cvmfs
build:gcc9:opt

00:12:31

passed #8475348
cvmfs
build:gcc9:opt:python3

00:09:36

passed #8475349
check-formatting

00:00:41

 
  Test
passed #8475350
check-copyright

00:00:56

passed #8475351
cvmfs
test:gcc8:opt

00:06:27

passed #8475353
cvmfs
test:gcc9:dbg

00:09:37

passed #8475352
cvmfs
test:gcc9:opt

00:06:10

passed #8475354
cvmfs
test:gcc9:opt:python3

00:05:46

passed #8475355
cvmfs
test_public_headers_build

00:02:09

passed #8475356
test-python3

00:01:17