[HTTP2StateMachine] test and fix HTTP2 go away

[dnadoba]

Motivation

• we should not crash if we close a stream after we have received a go away event

Changes

• rename AvailableConnectionContext to EstablishedConnectionContext to better communicate that the connection might actually not be available and already in the draining state
• add connectionID property to EstablishedConnectionContext because we might need to schedule an idle timeout if the connection is idle and we do not execute any new requests
• only lease streams if count is great then or equal to one
• add variouse tests to verify correct handling of go away events

[Lukasa]

Nice, this is a good cleanup and tidy fix.