Fix crash when WINDOW_UPDATE follows GOAWAY with buffered stream data

[manas-chaudhari]

When a GOAWAY frame removes streams from gun's map but does not inform cow_http2_machine, a subsequent WINDOW_UPDATE triggers send_data in the machine which finds buffered data for the removed stream and returns it, causing gun_http2 to look up the stream in its now-smaller map and crash with {badkey, StreamID}.

This happens regardless of whether the frames arrive in the same or separate TCP segments.

Fix: call cow_http2_machine:reset_stream/2 for each stream removed by the GOAWAY handler so the machine no longer holds buffered data for those streams.

Test failure before fix

** Reason for termination = error:{badkey,5}
** Callback modules = [gun]
** Callback mode = state_functions
** Stacktrace =
**  [{erlang,map_get,[5,#{}],[{error_info,#{module => erl_erts_errors}}]},
     {gun_http2,get_stream_by_id,2,[{file,"src/gun_http2.erl"},{line,1668}]},
     {gun_http2,send_data,6,[{file,"src/gun_http2.erl"},{line,1294}]},
     {gun_http2,send_data,4,[{file,"src/gun_http2.erl"},{line,1280}]},
     {gun_http2,frame,5,[{file,"src/gun_http2.erl"},{line,359}]},
     {gun_http2,parse,5,[{file,"src/gun_http2.erl"},{line,260}]},
     {gun,handle_common_connected_no_input,4,
          [{file,"src/gun.erl"},{line,1587}]},
     {gen_statem,loop_state_callback,11,
                 [{file,"gen_statem.erl"},{line,1395}]}]