Add NSEC3PARAM unit test, refactor zone.c

Add NSEC3PARAM unit test, refactor zone.c

Add unit test to ensure the right NSEC3PARAM event is scheduled in
'dns_zone_setnsec3param()'.  To avoid scheduling and managing actual
tasks, split up the 'dns_zone_setnsec3param()' function in two parts:

1. 'dns__zone_lookup_nsec3param()' that will check if the requested
   NSEC3 parameters already exist, and if a new salt needs to be
   generated.

2. The actual scheduling of the new NSEC3PARAM event (if needed).