HTTP streaming improvements

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Improves HTTP request and response streaming (#2404).

• Streaming request body skipping will only happen if the entire response has been sent before the user starts reading the request body (fixes body streaming doesn't stream #2393).

Note: Previously, streaming request bodies would be drained automatically by Vapor as soon as the response head was sent. This made it impossible to implement realtime streaming, like an echo server. With these changes, you have much more control over streaming HTTP while still preventing hanging if the request body is ignored entirely.

• Response body stream now supports omitting the count parameter (fixes body streaming doesn't stream #2393).

Note: Previously streaming bodies required a count and would always set the content-length header. Now, setting a count of -1 indicates a stream with indeterminate length. -1 will be used if the stream count is omitted. This results in transfer-encoding: chunked being used automatically.

• Response writer provides a better error message if stream is never ended (fixes unhelpful crash if response stream is never terminated #2390).

• Fixes an issue causing EOF framing to result in promise leak (fixes HTTP echo server crashes with unfulfilled promise if client uses EOF framing #2391).

• Response streams with determinate length that write too few or too many bytes will now result in an error (fixes illegal HTTP sent back #2392).

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These changes are now available in 4.11.0