Hook service must start before the first packet.

We have a general issue with hooks starting services by a post to the server IO service as this is IO service is polled so the services started after the first packet processing.

I suggest to add a poll at the end of loadConfigFile in ctrl_dhcp[46]_srv.cc to catch late issues e.g. bad configuration detected when the service is launched.

Hooks using deferred start: HA, LQ and soon RADIUS.