Distinguish between CAS prepare and propose failures

I spoke to a lot of users at the summit and had the feedback that the hardest part of building applications using LWTs is dealing with WriteTimeouts of type CAS.

Following up from https://issues.apache.org/jira/browse/CASSANDRA-8672 I think we should add a separate WriteType for timing out during the prepare phase assuming we can advise users to retry the operation or be confident it has failed as it won't be completed by a later SERIAL read or LWT.

We can't remove the ambiguity at the propose phase but this will remove one of the unknown cases.

cc slebresne bdeggleston spodxx@gmail.com

Happy to do a patch for this but assuming it will need a native protocol change when can we do it?