cache-file option writes format it cannot read back

Summary

Cache dumps produced by cache-file option cannot be read back on startup because they contain our own "meta RR types" like -URI.

BIND version used

BIND 9.16.20 (Extended Support Version) id:26db37f

I did not test v9.11 because we are unlikely to touch it before its planned EOL.

Steps to reproduce

1. Configure cache-file:

options {
    cache-file "/tmp/cache.db";
};

2. Use the resolver for a while
3. Shutdown the resolver
4. Attempt to start it again

What is the current bug behavior?

Named cannot start again because of the cache-file it produced moments ago:

/var/named/cache.db:4106: unknown RR type '\-URI'
loading configuration: unknown class/type
exiting (due to fatal error)

What is the expected correct behavior?

I guess it should load what it produced.

Possible fixes

Alternative "fix" is to rip out the option. It is documented as test-only and we do not use it in any test anyway.