Send periodical state messages from inactive clients

[pekam]

After the basic garbage collection functionality #4 is implemented, send a periodical state message to notify other peers about a client's state when it has not sent any messages for a while. This allows the garbage collector to clean the operation log even when one user has been inactive for a while.

Sending any message should reset the timer of sending the state message.

Instead of a timer, the state message could be sent when a set amount of messages have been received after the client's last own message. This could optimize the behavior to not send unnecessary messages when every peer is inactive, and on the other hand send the state message faster when the users are really active and garbage collection is useful more often.