Use a read lock when iterating over a hashmap

Use a read lock when iterating over a hashmap

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.