threadpool: do not reclaim thread by default

(or we will die because no more thread can be created)

Change-Id: I15f745d0f7e514940e956ce3fa58361bcff93a22
Reviewed-on: http://gerrit.aldebaran.lan:8080/18893
Reviewed-by: cgestes <gestes@aldebaran-robotics.com>
Tested-by: cgestes <gestes@aldebaran-robotics.com>