Use RCU for rad name

Use RCU for rad name

The RAD/agent domain is a functionality from RFC 9567 that provides
a suffix for reporting error messages. On every query context reset,
we need to check if a RAD is configured and, if so, copy it.

Since we allow the RAD to be changed by reconfiguring the zone,
this access is currently protected by a mutex, which causes contention.

This commit replaces the mutex with RCU to reduce contention. The
change results in a 3% performance improvement in the 1M delegation
test.