Resolve "ThreadSanitizer: data race in dns_tsigkeyring_dump"
The 'dns_tsigkeyring_t' structure has a read/write lock to protect its 'keys' member, which is a 'isc_hashmap_t' pointer and needs to be protected.
The dns_tsigkeyring_dump() function, however, doesn't use the lock, which can introduce a race with another thread, if the other thread tries to modify the hashmap.
Add a read lock around the code, which iterates over the hashmap.
Closes #4328 (closed)