Fix a date race in qpcache_addrdataset()

Fix a date race in qpcache_addrdataset()

The 'qpnode->nsec' structure member isn't protected by a lock and
there's a data race between the reading and writing parts in the
qpcache_addrdataset() function. Use a node read lock for accessing
'qpnode->nsec' in qpcache_addrdataset(). Add an additional
'qpnode->nsec != DNS_DB_NSEC_HAS_NSEC' check under a write lock
to be sure that no other competing thread changed it in the time
when the read lock is unlocked and a write lock is not acquired
yet.