re support the StopIteration status

[wbpcode]

We converted all StopIteration to StopAll at #95.

It make the wasm filter couldn't work in some typically scenarios, for example, custom auth may require to parse body to complete the authentication, but return Continue when calling onRequestHeaders will proxy headers to uptream before we complete the authentication, return StopAll will prohibit the wasm to access body.

[mpwarres]

#143 discusses the same (or similar) issue, suggest deduping and reviving the discussion there, unless there is a different aspect to this issue. @wbpcode WDYT?

[wbpcode]

yea, it's the same issue. This is a very important feature for a bunch of filters.

Without it, we even cannot set the content length correctly after mutate the body. It's weird.