Use async name resolution on Linux

Edit by @antonfirsov:

this issue has been reopened to track the missing feature, and the related disabled test (see #70089). We should not use getaddrinfo_a as proposed by the original opening comment, since it turned out to be sub-optimal (see #48666).

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We should use getaddrinfo_a on Linux for async name resolution. It supports cancellation as well, bringing us in line with Windows support. See sample code for a usage example.

Note: Today we do async over sync on Linux.