Flushing entries on Prepare

[kevin-harrison]

Right now we flush entries on receiving a Prepare message. There is nothing wrong with this, but it could be optimized. If we in general only send Accepted after flushing then actually this is not strictly necessary. (any chosen entry must be flushed among a majority and if we haven't flushed it, then we will get it in the AcceptSync). To prevent flushing stuff that will anyway get overwritten, we should perhaps do some checks here first. The most basic check is if the leader is more updated than us, we don't flush. There are probably more sophisticated checks we can do.