Use TLS for plugin_weighted PRNGs
Use TLS for dnsio_tcp thread context
Remove threadnum from plugin resolve() callback args

... and related fallout.  Basically now that TLS is
available, this argument and its storage in
dnspacket_context_t is superfluous.

For plugins which really need access to the thread number at
resolve() time (although honestly, TLS should suffice on its
own), a simple solution is to store the threadnum given in
iothread_init() into a TLS var and access it from resolve().
Use TLS for dnspacket context, split stats, etc
some signed-vs-unsigned cleanup
Re-arrange rrset gen struct layout
s/unsigned int/unsigned/
dnsio_tcp: slightly optimize RCU here as well

Rather than flipping offline/online around entering
the eventloop core every time, turn it on just before
processing a whole query (at most once, even if a loop
iteration processes several), and turn it back off
before eventloop re-entry if it was turned on.
silence djb dir error at startup