[store/meta]: add disk backed raft-state impl

[drmingdrmer]

Summary

Persist raft state, which includes id, current_term and voted_for on disk.

This is the first step to impl the disk backed raft meta.
The next step is to impl disk-backed log and state machine.

Part of #271

[jovany-wang]

Sorry for that I didn't understand the purpose to persist the metas.
In general cases, it's totally enough to persist log entries and user-defined state machines. Is there any context I have missed?

[drmingdrmer]

There are some other information must be persisted, according to the raft paper, including the current term, and the leader a node has voted for.

E.g. if the voted_for get lost when restarting a raft node, there will be two nodes believes they are the legal leader.
Then the log get messed up:

R1 elect       become_leader                                  write_log1
   |                                                          |
   v                                                          v
R2 vote_for_R1               crash vote_for_R3                log1?log2?
                                   ^                          ^
                                   |                          |
R3                                 elect        become_leader write_log2
-------------------------------------------------------> time

[jovany-wang]

There are some other information must be persisted, according to the raft paper, including the current term, and the leader a node has voted for.

E.g. if the voted_for get lost when restarting a raft node, there will be two nodes believes they are the legal leader.
Then the log get messed up:

R1 elect       become_leader                                  write_log1
   |                                                          |
   v                                                          v
R2 vote_for_R1               crash vote_for_R3                log1?log2?
                                   ^                          ^
                                   |                          |
R3                                 elect        become_leader write_log2
-------------------------------------------------------> time

OK, I see.

[drmingdrmer]

closed by #1056