c-ares / c-ares / #1285

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UDP write may fail indicating host isn't reachable (#821)

UDP is connectionless, but systems use ICMP unreachable messages to
indicate there is no ability to reach the host or port, which can result
in a `send()` returning an error like `ECONNREFUSED`. We need to handle
non-retryable codes like that to treat it as a connection failure so we
requeue any queries on that connection to another connection/server
immediately. Otherwise what happens is we just wait on the timeout to
expire which can greatly increase the time required to get a definitive
message.

This also adds a test case to verify the behavior.

Fixes #819
Fix By: Brad Houes (@bradh352)

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