Backported the hp_core_spec.
Added maximum TTL enforcement also to MDNS client used in hp. By
default it's same as the one in hp_core. Now it's possible to have
mdns records also expire in a limited amount of time.
Merge branch 'vancouver' of github.com:fingon/hnet-core into vancouver
Added is_empty check.
Merge branch 'vancouver' of https://github.com/fingon/hnet-core into vancouver

Conflicts:
	hp_core.lua
Using shorter timeouts for demo purposes