Simplify synchronization logic in task_group_context (#462)

The current synchronization in task_group_context made by more then one atomic variable. This code hard to debug and each ThreadSanitizer report in this code usual require unreadable workaround to fix it. To improve correctness and readability of this code it was decided to rewrite synchronization using mutex.

Signed-off-by: Ilya Isaev <ilya.isaev@intel.com>