raft: fix send

send should not attach current term to msgProp. Send should simply do proxy for msgProp without
changing its term. msgProp has a special term 0, which indicates that it is a local message.
/cc @dterei 
yea, please move it. Also the prose needs to be cleaned up a bit:

```
Do not attach a term to msgProp. Proposals are a way to forward to the leader and should be treated as a local message.
```