Add a check to see if room members are receiving keys from the captain

[Mandragorian]

I added a timer that checks periodically if we are still receiving messages from the captain.

Specifically it has a period of 2 minutes more than the rekeying period. If in the mean time we have received a key transport from the captain the timer is unhooked and nothing happens. If we havent then the still hooked timer triggers and a message is printed in the channels buffer informing the user that something has gone wrong.