protocols/request-response: Consider introdcuing InboundFailure::ConnectionClosed

[mxinden]

Today the request response NetworkBehaviour might not always emit an InboundFailure for all inbound requests that failed to be answered. More specifically in case the connection to the peer closes after RequestResponse::send_response has been called the response will be dropped but no InboundFailure event will be emitted.

rust-libp2p/protocols/request-response/src/lib.rs

Lines 143 to 152 in 7985c81

	/// An inbound request failed.
	///
	/// > **Note**: The case whereby a connection on which a response is sent
	/// > closes after [`RequestResponse::send_response`] already succeeded
	/// > but before the response could be sent on the connection is reflected
	/// > by there being no [`RequestResponseEvent::ResponseSent`] event.
	/// > Code interested in ensuring a response has been successfully
	/// > handed to the transport layer, e.g. before continuing with the next
	/// > step in a multi-step protocol, should listen to these events.
	InboundFailure {

Also see paritytech/substrate#7696 (comment).

Consider tracking inbound requests in the NetworkBehaviour in order to be able to detect the above scenario and emit an InboundFailure::ConnectionClosed event.