Crash caused by enabling dedicated DNS thread

When enabling ``CONFIG proxy.config.dns.dedicated_thread INT 1`` we're observing a crash because apparently the continuation is called back while still on the DNS thread. This becomes a problem because when using shared server sessions 2 (per-thread sessions) it relies on a thread local that only exists on ET_NET / ET_SSL threads. The fix for this shouldn't be bad, we just need to make sure we pop over to a ET_NET thread when calling back the continuation.