Transport observer should be aware of accepted inactive channels

[tkountis]

We recently added support for 'stop-accepting' connections on the server through an API.
When this is used, it allows connections to establish (3-way handshake) and then remain in the accept_queue (i.e. SO_BACKLOG) until they are accepted by the server. This of course depends on the setting for the backlog.
Connections waiting to be accepted may become inactive for various reasons e.g. client timeouts. When we accept them we check the state of the channel before making any progress with it and drop them otherwise. Its worth adding visibility for this behaviour in TransportObserver.

discussion: #1741 (comment)