Ability to shed traffic with HTTP/2

[masaori335]

Migrate a ticket from JIRA. Details in TS-3864.

[masaori335]

From #1673, it looks like @zizhong has some ideas:)

[mlibbey]

This would also be a great feature addition to the header rewrite plugin -- eg, allowing it to send a GO AWAY frame as an action.

[maskit]

The feature is implemented, and "Connection: close" in response header triggers a GOAWAY frame. It'll be available on 8.0.