MQTT module has static DNS handling

[nwf]

The MQTT module nominally can juggle multiple clients, each with an associated lightuserdata object, and yet it contains "static ip_addr_t host_ip; // for dns". So in practice it can only be doing one DNS resolution at once and may end up crisscrossing resolution results between multiple client objects.

I think I would like to make a breaking change to the C module: no DNS support for the hostname. We can (I will) provide a Lua wrapper that does DNS resolution and hands the address down to the mqtt module. (As with #2819, I want to keep the C as simple as possible; for MQTT, keeping the state machine in C is fine, I guess, since it's already there and working.)

[nwf]

Worked around, sort of, mostly, by part of #3360. See #3063 for motion against removing DNS handling from C and workaround adopted in TLS. However, at the time of this writing the refcount work necessary has not been done and so mqtt remains a source of badness.