Accept PING frames even when fully quiesced

[clintonpi]

Motivation:

When the connection is fully quiesced, an endpoint responds with a connection error when it receives or tries to respond to a PING frame. This behaviour is unnecessary.

Modifications:

• Adjust HTTP2ConnectionStateMachine to successfully receive and send PING frames even when the connection is fully quiesced.

Result:

When the connection is fully quiesced, an endpoint will treat receipt and sending of a PING frame as it would when the connection is active.

[clintonpi]

Oops. Yes, of course.

[Lukasa]

@swift-server-bot test this please