raft: add Configure

Configuration change
let me elaborate it.
For now, when propose configuration, server calls `Configure` and gives `$data`. To see whether it happens, server keeps grep things from `Ready()` channel, and checks `CommittedEntries` in the Ready struct. `CommittedEntries` is composed of Entries in the format `{Type: , Index: , Term: ,Data: $data}`.
If we give id and context and let s.Node marshal it, server still needs to unmarshal it. i think input and output are asymmetry then.