real default value (as described everywhere)
add check_delay option
when check_delay < 0 (unit tests), there is no periodic callback "manage_watchers" any more
check_delay < 0 for tests (by default)
async rewrite of this test (to avoid problems with check_delay < 0)
check_delay < 0 by default
check_delay < 0 by default for tests
check_delay > 0 required here
Merge pull request #605 from thefab/fix599

Fix599