[IO] slightly change the API to prevent parallel modification of an io_uring's SQ

With this change the globalCompleter does not support partial completions
and will signal a Future immediately and thus never modifying the io_uring's
SQ.
As consequence we can't give guaranties about completion if either {read, send, write}_all
is specified.
11 jobs for worker_exclusive_uring_weak in 1 minute and 27 seconds (queued for 1 second)
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