test/multi_stress_watch: use condition_variable for level triggering

test/multi_stress_watch: use condition_variable for level triggering

* use mutex+condition_variable instead of POSIX semaphore, for
  couple reasons:
  - for using POSIX semaphore, we should set the initial value:
    before this change, sem_init() is not called, so the initial
    value of the semaphore is not set.
  - use condition_variable::wait_for() instead of TestAlarm. simpler
    this way. and avoid hiding the magic numbers -- if we choose to
    live with them, make them more visible.
  - use RAII to avoid sem_init() and sem_destroy()
  - use a member variable for being level triggered. as notify()
    could be called before wait() is called.
* define 10000 using a constant variable: NR_ITERATIONS
* remove unused `#include <sstream>` and using declaration.
* use cout for printing informative messages

Signed-off-by: Kefu Chai <kchai@redhat.com>