MEDIUM: dns: Stick the TCP nameserver tasks to the resolvers' thread

MEDIUM: dns: Stick the TCP nameserver tasks to the resolvers' thread

The tasks handling the TCP nameservers (task_req, task_rsp, task_idle)
are created with no thread affinity, but with thread groups having their
own file descriptors table, it has to run on the same thread as the
one who created the socket, or maybe the other thread will not be able
to use the socket.
In practice, it was not a problem, because the scheduler will wake the
task on the running thread. But that may change in the future, so make
sure we always run on the current thread from start to finish.