Cthreads: implement task queue per thread with task stealing (#528)

Cthreads classes:
- Notification queue : Manages tasks: tries or forces popping and pushing tasks.  
- Task system : manages the notifications queues; controls which queue to pop from/push to; controls spinning on queues if necessary; manages creating/joining threads. Is a singleton.
- Task group : manages synchronization on a group of tasks. 

Operation: 
- Each thread has an associated queue
- Task system _tries to_ push tasks in one of the available queues. If it is unable to acquire a lock on a queue, it tries the next in a round robin fashion. After it loops all queues if it still hasn't successfully pushed the task, it spins on a single queue until lock is acquired and task is pushed. 
- Task system _tries to_ pop a task from the calling thread's queue. If it is unable to acquire the lock, it tries to steal the task from another thread's queue, in a round robin loop. If it is still unable to pop a task, it spins on its the calling thread's queue until the lock is acquired. 
- Task group keeps a counter for the number of tasks in the group which it increments/decrements when calling push/pop on the task system. The counter is used to know when all tasks in the group have been executed. 

Unit tests: 
- Basic tests to pop/push from notification_queue, task_system and task_group. 
- Simple tests for deadlock 
- Simple tests for correctness 

Benchmark: 
- Benchmarks performance for various task sizes