getaddrinfo can hang for many seconds when the system stub resolver
misbehaves; running it inline on the event loop froze every other
request, IPC, and curl socket event for the duration.
New PendingSystemResolution coalesces concurrent lookups for the same
name, dispatches the call to a Threading::ThreadPool worker, and
deferred-invokes the result back to the originating event loop. Each
caller of lookup() gets its own Core::Promise so concurrent
when_resolved/when_rejected handlers can't clobber each other; the
pending state fans out to every joined caller on completion.
Workers issue A and AAAA in parallel on separate sockets to sidestep
the systemd-resolved AAAA-drop bug, and resolve the user's promise
after the first side returns records (with a 50 ms RFC 8305 grace
window for the other side).
Adds Core::Socket::AddressFamily and an optional parameter to
resolve_host so workers can request A or AAAA specifically.
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