threadpool: add stop/reset methods, fix destruction

when calling stop or the destructor:
- all pending task are removed.
- a bool is set to block schedule from adding new tasks

Change-Id: I8a04b1c28d80129966f2afaaafca27916e4f5a1d
Reviewed-on: http://gerrit.aldebaran.lan/25094
Reviewed-by: ncornu <ncornu@aldebaran-robotics.com>
Tested-by: ncornu <ncornu@aldebaran-robotics.com>
eventloop: threadpool: default to 100 threads, do not accept new tasks when stopping

Change-Id: I9b9f6052f2b5245aba3dd3c7ab61776f2d430ccc
Reviewed-on: http://gerrit.aldebaran.lan/25095
Reviewed-by: ncornu <ncornu@aldebaran-robotics.com>
Tested-by: ncornu <ncornu@aldebaran-robotics.com>