Stream: speed up TCP peer recovery. Previously, an unavailable peer was considered recovered after a successful proxy session to this peer. Until then, only a single client connection per fail_timeout was allowed to be proxied to the peer. Since stream sessions can be long, it may take indefinite time for a peer to recover, limiting the ability of the peer to receive new connections. Now, a peer is considered recovered after a successful TCP connection is established to it. Balancers are notified of this event via the notify() callback.