Time-restricted asynchronous DNS [WIP]

[splitice]

Behaviour Changes:

• Initial packet will fail if DNS is slow (20ms) to respond the DNS request will continue to be processed in the background
• Any further packets after the first will only immediately check the status and will immediately fail if the DNS request has not completed yet
• Requires pthreads (should we gate this and support the older sync method if the system does not support pthreads e.g windows?)
• If a DNS lookup fails existing known host addresses are kept (I think this is better behaviour)

Remaining:

• Code cleanup: Perhaps this should live in resolver.c as str2addrinfo_async & str2addrinfo_sync ?
• Style

[splitice]

@gsliepen @hg Any idea why the proxy test fails?

I can't see why myself. I purposefully did not change the proxy.c dns side of things as it should be a locally cached DNS entry and potentially better to be blocking in nature.

INFO:proxy.py:FINISHED TEST: HTTP CONNECT proxy
ERROR:proxy.py:Uncaught exception
Traceback (most recent call last):
  File "/__w/tinc/tinc/test/integration/proxy.py", line 464, in <module>
    check.greater(proxy_stats["tx"], 0)
  File "/__w/tinc/tinc/test/integration/testlib/check.py", line 64, in greater
    raise ValueError(f"value {value} must be greater than {than}")