Server->client messages not always delivered immediately

[srottem]

If the client is "between" polls, then a message for immediate delivery
will not be delivered to the client until the end of the next long-poll
timeout - a 10 second delay.

If the turn around for the old long-poll response to be returned, cross the
network, be processed by the client, a new long-poll request to be created,
cross the network, and processed by the server is (say) 0.1s, then there is
a gap of 0.1s every 10s where this may happen - or around 1% of the time.

Note; this happens because Client.FlushQueue() will only send the messages
if there is a current long-poll active. If there isn't, the next long-poll
opened by the client isn't flushed immediately.