Sender connections to edge router that connect 'too soon' never get credit

A stand-alone self test was extracted from system_tests_edge_router. Running the test involves starting the routers and then running the client. If this is repeated in a script then there must be a fairly long delay between starting the routers and running the client.

If the delay is 6 seconds then the client sender seems to run fine every time.

If the delay is 4 seconds then the client sender routinely (half the time?) fails to get an on_sendable event as no credit is ever issued by the edge router. A protocol trace to be attached.