Read-Only MIB for 'fixed' DNS names.
when a DNS entry is fixed, a refresh operation is invoked periodically,
to make sure the entry is kept 'warm' in the DNS cache.
if the refresh operation fails, the DNSCL will enter a polling state,
and will retry to resolve the entry again in constant intervals.
polling-interval duration for unresolved entries is configurable
by MIB scalar 'rlDnsClMinPollingInterval'.