Timeout hint for the round

[fjarri]

In some protocols different nodes need different number of messages to finalize the round. Sometimes none. Some rounds take more time than others, and the protocol level has this information. It would be nice to provide some kind of hint for the user of Session to adjust the timeout accordingly.

Note also that it's possible that some subset of nodes generates echo messages, so they will need an echo round, while other nodes switch to the next round right away. The latter nodes may still need to know that an echo round is happening so they have to wait more.