Actually disconnect on timeout

[Drahflow]

lymph.core.connection.Connection pings the endpoint regularly to determine whether it is still live. __init__ takes a unresponsive_disconnect parameter, which was however ignored so far. Similarly, the idle_disconnect parameter did not have any effect.

This commit uses thees two parameters to do what they advertise, i.e. close the connection once the respective timeouts have been reached.

To this end, the greenlet management logic was also changed, such that the monitoring greenlets now commit suicide when the connection status == CLOSED instead of being explicitely killed from close(). Without this change, the monitoring greenlet would inadvertently kill itself, leading to (very nonobvious) dead code and leaving a half-dead connection object still registered with the server.

(edited as requested by @mouadino)