Stop saving anonymous functions to disk in ns_audit

Stop saving anonymous functions to disk in the event of a ns_audit shutdown (in gen_server terminate hook) and re-reading them on boot. Reason being, that if the ns_audit beam file changes in between saving and reading, we will fail with a "badfun" error. See: https://issues.couchbase.com/browse/CBSE-12404 and the conclusion to understand more. 

It's probably relatively rare to hit this issue, in practice, but it's still a major issue and something we need to deal with.