Fix locking bug in high-level stealing.

The lock was not released when an split_task was executed. We did a dirty fix by allowing the method to unlock the lock itself, but this has to be done cleaner in the future.
4 jobs from tbb_like_stealing in 3 minutes 16 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #2735
build_cmake

00:36

 
  Test
passed #2736
run_tests

00:34

 
  Sanitizer
passed #2738
run_address_sanitizer

01:07

passed #2737
run_thread_sanitizer

00:58