add checks which avoid processing messages meant for an old hand

there seems to be a bug where a message from a client is sent
to the server only after the current hand ended and a new hand
has started - leading to a situation where east has 15 tiles
at the beginning of a hand. This commit does not fix that, but
it hopefully throws an exception if a message is received that
is not meant for the current hand.

the debug output --showtraffic suppresses displaying this token
which is sent with every message.