Wake up launcher when enabling a subscription

Otherwise one would have to wait up to DEFAULT_NAPTIME_PER_CYCLE until
the subscription worker is considered for starting.

There is a small race condition:  If one enables a subscription right
after disabling it, the launcher might not have registered the stopping
when receiving the wakeup signal for the re-enabling.  The start will
then not happen right away but after the full cycle time.

Author: Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>