fix: dev: Fix a data race in dns_zone_getxfrintime()

The dns_zone_getxfrintime() function fails to lock the zone before
accessing its 'xfrintime' structure member, which can cause a data
race between soa_query() and the statistics channel. Add the missing
locking/unlocking pair, like it's done in numerous other similar
functions.

Closes #4976

Merge branch '4976-zone-xfrintime-data-race-fix' into 'main'

See merge request isc-projects/bind9!9591