Added opt-in locking of first detected async backend

[agronholm]

Changes

Prompted by #543, this adds an optimization, enabled by the ANYIO_BACKEND environment variable, for skipping the costly sniffio automatic async backend detection after the first call. Whatever backend is detected is assumed to be running on every call since. This avoids the getpid() system calls in this extremely often called function.

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* Fix big bad boo-boo in task groups (#123 <https://github.com/agronholm/anyio/issues/123>_; PR by Yourname)

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