Fix shutdown for HTTP/1.1 pipeline

[dmitrivereshchagin]

Draft fix for #1582.

• Call cowboy_stream:terminate/3 once for each stream (do not terminate streams from cowboy_http:initiate_closing/2).
• Prevent next response to be written after "close" option has been sent (check if the last stream ID is reached in cowboy_http:stream_next/1).

[essen]

Isn't a better fix to terminate immediately all streams except the one currently executing (lists:keytake would give us that stream and the remainder to terminate) and then store only that remaining stream in streams in the state.

In any case a test case reproducing the issue should come first. http_SUITE:graceful_shutdown_connection already does some pipelining but the test can likely be enhanced to ensure we get the correct response.

[dmitrivereshchagin]

Thanks for your suggestion. Please take a look again.

[essen]

Please rebase onto current master so that CI runs properly. I am planning to do a release with this and other things soon.

[dmitrivereshchagin]

Rebased onto master.

[essen]

This was merged with minor tweaks. Closing, thanks!