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.