Resolve "Statistics XML rendering does not seem to protect against concurrent changes" [v9_18]

There were a number of places where the zone table should have been locked, but wasn't, when dns_zt_apply was called.

Added a isc_rwlocktype_t type parameter to dns_zt_apply and adjusted all calls to using it. Removed locks in callers.

(cherry picked from commit f053d5b4)

Closes #3468 (closed)