io_uring: mark callbacks as noexcept
io_uring: mark callbacks as noexcept

Summary: Rather than forcing the IoUringBackend to deal with exceptions - push it to the callbacks like other async callbacks (eg folly::AsyncTransport)

Reviewed By: praihan, ycking21

Differential Revision: D37954822

fbshipit-source-id: 29c8079e4f8e5a40f6be02cd93e2b0e128804c55

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