Stream: speed up TCP peer recovery.

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.