asyncd: fix dataloss bug when sleeping too little

When sleeping too little, the code just discards every plugin, and reverts to
sleep $timeout sec.

Note, it was hot-fixed by f47da1da171b60d06753321a6e60686948940869, so this
commit completes the latter for a much proper fix.

Thx to trippeh on IRC for showing me the bug *very* clearly.