Pairing messages not send when player was unavailable at round start

[hopffgam]

In the following situation no pairing message is sent:

1. Team player A gets unavailable before round start.
2. No alternate is found until round start
3. At round start the opponent B of player A gets a message from chesster that we are still searching for an alternate
4. Before an alternate is found, player A decides to switch to available and play the game. The alternate search is now stopped but A and B do not get a pairing message.

Expected: there should be a pairing message in the above scenario when player A goes available in step 4.

An additional consequence of this issue is that if the game does not get scheduled within 24 hours we assume that both players are unresponsive which is wrong as without pairing message channel we cannot even check the responsiveness.

[hopffgam]

#261 seems to be linked to this one as well.