Improve const correctness
@bind-team, this is 3 years old review of BIND code by Errata Security: https://blog.erratasec.com/2015/07/a-quick-review-of-bind9-code.html and he is right.
We need to start using
const more extensively and internalise it when writing new code and doing reviews.
It is a long term goals, so we will just keep this open, and I'll list major components that should be reviewed: