All nodes should transition to Follower if term > currentTerm

[colin-scott]

From the raft paper:

• If RPC request or response contains term T > currentTerm:
set currentTerm = T, convert to follower (§5.1)

This is not implemented in all of the akka-raft states for all messages. It does seem to be implemented for AppendEntries (Follower, Leader, Candidate) but there are still three remaining issues:

• It is not implemented for other message types.
• Upon receiving AppendEntries from a higher term in Candidate state, the actor does convert to Follower state, but it does not update its Term
• Upen receiving AppendEntries from a higher term in Leader state, the actor does convert to Follower state, but it does not update its Term

[colin-scott]

This issue is distinct from #46. #46 is the opposite condition:

if term < currentTerm: 
  ignore