This (hopefully) fixes a memory leak that caused active and
pending ypdata_t objects to be not freed up on server shutdown
or config reload.

To testers: please enable yp submission and restart and SIGHUP
your instance several times with loglevel set to warning or higher.
Each time after shutdown or SIGHUP check your log for one of the
following two warnings:
 * 'active ypdata not freed up'
 * 'pending ypdata not freed up'

Those warnings must not be in the log. If you spot one please report.