chunker: infinite loop in the Rabin chunker

At Infura we got some of our nodes ending in an infinite loop in the Rabin chunker, with the CPU at 100% until something bad happen (the context doesn't reach there so loop keep spinning even though the request is long gone).

Here is a profiling:

Original requests are `add` with the following parameters: `[chunker=rabin-262144-524288-1048576,encoding=json,hash=sha2-256,inline-limit=32,pin=false,progress=true,stream-channels=true,]`. Some also have `trickle=true` but that doesn't seem to matter.

Profiling lead to an infinite loop in https://github.com/whyrusleeping/chunker/blob/master/chunker.go#L209-L336 but it's unclear to me what is happening. `if err == io.ErrUnexpectedEOF { err = nil }` is suspicious but that kinda fly over my head.