RematchOffers keeps an offer open for ten minutes after a game nobody
left. A request goes to the opponent; when both have asked the next game
of the series starts with the other player moving first. Requests end on
no, withdrawal, a minute without answer, going offline or looking for
another game. The bot accepts at once.
Every game starts a series; a game created in a series gets its seat to
move first. Clients get the series and the first seat with the game, the
end message the series with the ended game; snapshots keep the series.
