Deprecate the "glue-cache" option (not the feature!)
As discussed in !4122 (merged), the rationale for introducing the glue-cache
option was to have a safety switch readily available in case the
glue cache feature turns out to cause problems. No problems have been
reported with that feature since its introduction in BIND 9.12 and thus
the glue-cache option should be deprecated and ultimately obsoleted;
glue cache should be permanently enabled in the long run.
According to the policy for removing named.conf options that we
published last year, we should first deprecate the option in
9.17/9.18. As the code implementing that option is not (at least for
now) harmful to keep, we should only fully obsolete the option and
remove the code handling it in 9.19/9.20.