ensure lease expirations never fire too early

I noticed in logs from CI that a certain LeaseExpiration was delivered too early, indicating Lwt_unix.sleep can return too early.
( Sep 27 15:31:11 7028: (paxos|debug): sturdy_1: lease expired (8.0 passed, intended 8.0, diff -0.000040)=> injecting LeaseExpired event for 11 )
This PR should fix that problem.