changes for SpdyPersistentUrl:

 - rename the stream_closed ivar to stream_is_invalid
 - use this on ping failed to indicate that we really should reconnect when that is called
 - upon both stream close and failed ping, schedule a reconnect, don't do it immediately
 - add a resetState method which resets both stream_is_invalid and num_reconnects
 - use this resetState method everywhere that we were resetting num_reconnects, to make sure that stream_is_invalid values do not linger
add some logging to SpdySession, in invalidateSocket