Add too_many_resets debug_data to the GOAWAY we send

[seanmonstar]

There was a new GOAWAY added in hyperium/h2#668. It would be helpful to send some extra debug data in the GOAWAY so that peers can know they hit a limit. This is now easier thanks to #3200.

• Add a library_go_away_data() constructor.
• Add a too_many_resets message to that error.
• Update the unit test that it received the debug data.
• Remove the extra #[unstable] introduced in Add internal serialization of debug_data to HTTP/2 GOAWAY frames #3200 from src/server and src/proto/connection, and the other unit test.