Remove first_present_after_reset.

It shouldn't be needed

Signed-off-by: Axel Davy <axel.davy@ens.fr>
Improve threadpool code for less overhead

Signed-off-by: Axel Davy <axel.davy@ens.fr>
Implements thread_submit

Signed-off-by: Axel Davy <axel.davy@ens.fr>