Investigate addition of a "DcpCommitWithValue"

See https://issues.couchbase.com/browse/MB-34542?focusedCommentId=338119&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-338119.

One "nice" solution for this and the edge case where we allow a mutation to complete a prepare would be to instead send a commit with a value. This is a chunk more work to add so want to investigate this post-beta because simpler solutions for the described problem are workable. In this case we would only send the commit with the value when necessary (following the same logic where we send a mutation instead of a commit on the active node). This would allow us to remove the edge case code that accepts mutations and allows them to overwrite prepares on the replica. This would result in more code, but I think that it could be simpler and more manageable.