query context management issues in dighost.c

For the reference, the _cancel_lookup() function iterates through the lookup's queries list and detaches them. In the ideal scenario, that should be the last reference and the query will be destroyed after that, but it is also possible that we are still expecting a callback, which also holds a reference (for example, _cancel_lookup() could have been called from recv_done(), when send_done() was still not executed).

The start_udp() and start_tcp() functions are currently designed in slightly different ways: start_udp() creates a new query attachment connectquery, to be called in the callback function, while start_tcp() does not, which is a bug, but is hidden by the fact that when the query is being erroneously destroyed prematurely (before _cancel_lookup() is called) in the result of that, it also gets de-listed from the lookup's queries' list, so _cancel_lookup() doesn't even try to detach it.