Obsolete the "glue-cache" option

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. Mark the "glue-cache" option as obsolete and remove all code related to it.

Note: this causes the glue cache feature to be permanently enabled, not disabled.

Closes #2147 (closed)