Make splice atomic

[badeend]

Partly a PR, partly a question: what is supposed to happen with the already read data in case the write within splice fails?

Currently, wasmtime throws the data away.

Can we make it such that the splice implementation first reads into a per-stream stash and only then attempt to write it into the destination output stream? Future reads then first check if there is any data in its local stash, before calling the backing implementation.

I think this is in line with Linux' splice and sendfile

[pchickey]

Yes, we should follow the lead of splice(2) and not throw away read bytes if a write fails. I was lazy about doing that correctly in wasmtime's implementation, but the behavior as it exists today is not desirable.

@guybedford what is the jco behavior for splice?