[narwhal adpater] Add backoff mechanism to consensus adapter

[gdanezis]

We add a mechanism to estimate the average delay in consensus (using a weighted average with weight 1/2 is the last sequenced message / timeout) and then adapt the time we wait for a message to get sequenced to be the average delay + the max delay (instead of just the max delay). This avoids the churn of constant timeouts + re-submissions at times when consensus is already slow.