In +nssearch mode dig starts the next query of the followup lookup
using start_udp() or start_tcp() calls without waiting for the
previous query to complete.
In UDP mode that happens in the send_done() callback of the previous
query, but in TCP mode that happens in the start_tcp() call of the
previous query (recursion) which doesn't work because start_tcp()
attaches the lookup->current_query to the query it is starting, so a
recursive call will result in an assertion failure.
Make the TCP mode to start the next query in send_done(), just like in
the UDP mode. During that time the lookup->current_query is already
detached by the tcp_connected()/udp_ready() callbacks.
Closes #3144 (closed)