previously cryptolib random function was used to generate message id,
this works well but it is slow especially when the entropy is low,
replaced with cryptographically safe prng ISAAC

the ccan directory is going to be used in the future, as it's include
structure makes it easy to embed C snippets instead of reimplementing
them

- cache is now fully optional, resolution works without cache as well
- proper mempool per each request, not mixed
- zone cut resolution doesn't need any per-context memory
- now it's possible to do qname minimization
- rplan keeps tabs on resolved queries
- added documentation etc.

TODO:
pruning, better removal, periodic cleanup, configuration

Slow start evaluates all NS in the preference list, using mean as
a sorting function. Invalidated NSs are pushed to the back of the
preference list.