move some design docs to rfcs

[siddontang]

We have written many design docs internally, and we need to public them to let users know TiKV deeply.

• [ ] Raft merge
• [x] Batch split #6
• [x] ReadPool optimization
• [ ] Remove region meta from etcd
• [ ] Distributed GC
• [ ] Batch message
• [ ] Coprocessor chunk
• [ ] Raft Local reader
• [ ] Region heartbeat optimization
• [ ] Follower/Leader read
• [ ] Scheduling based on priority
• [ ] API check for compatibility rolling update
• [ ] Titan - a new engine for RocksDB
• [ ] Titan GC
• [ ] Joint consensus
• [ ] lightning
• [ ] PD simulator
• [x] PD schedule limit
• [ ] Threaded Raftstore
• [ ] Failpoint
• [ ] Region Consistency check

[Hoverbear]

Please label all PRs for this with the "Migrated RFC" tag so our team can know that it has already been accepted/implemented.

[tisonkun]

cc @siddontang I'd like to see the design document of -

• Joint consensus
• Raft Local reader
• Follower/Leader read

If any google docs or internal docs can be shared here, I'm glad to port them into English design document for archiving.