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
• Batch split Add batch split #6
• 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
• 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.