Posix: Free the allocated memory after deleting a task or ending the scheduler (...

Posix: Free the allocated memory after deleting a task or ending the scheduler (#181)

* Posix: Free Idle task resources after ending the scheduler

In case of using Posix simulator and ending the scheduler, it does
not free the resources allocated by the idle task. This
causes the memory checkers (Valgrind, Address Sanitizers, ..) to
complain.

* Posix: Free the condition variable memory in the correct place

In case of deleting a task from another task, the deletion happens
immediately and the thread is canceled but the memory allocated by
the task condition variable is not freed. This causes the memory
checkers (Valgrind, Address sanitizers, ..) to complain.

* Posix: End Timer thread and free its resources after ending the scheduler