Bump Raft protocol version default to 3

[slackpad]

Meant to do this in 0.9.0, but we should do this as soon as possible. This enables the rest of the Autopilot features by default, and handles server identity more robustly for Raft peer management. We will need to document that Consul requires version 0.8.0 or later, otherwise you need to set this back (need to confirm exact details and document this in change log and upgrade guide).

[slackpad]

Actually the peers.json aspect of this might warrant pushing this off to the next major release - https://www.consul.io/docs/guides/outage.html#manual-recovery-using-peers-json. This could cause a fair bit of work for some folks who have automated recovery.

[slackpad]

Working on this - since AP won't promote an even number of servers to voter we need to fix up about 10 unit tests, so this'll take another hour or two to finish off.