Network requester should periodically remove stale proxies

[jstuczyn]

Right now when a proxy is started, it is put into a separate task on the network requester end. It is only removed when socket is explicitly closed on either end (client or network requester). However, if client closed his browser tab that was using a proxy, such message would never get sent and hence network monitor would have garbage, from its point of view, data stuck in memory.