fix ws-conn will to work as intended

[djholtby]

• changes ws-conn-read-thread to hold a reference to ws-conn weakly, so that the ws-conn can be garbage collected even if it is still active
• the will closes the connection if it is still closed (so the client will not see a broken-pipe error, but proper websocket disconnect)
• the thread stops if the ws-conn is collected (break is not needed in the will)