roadmap: update TiKV roadmap

Via: tikv/tikv#3641

ROADMAP.md

@@ -39,28 +39,45 @@ This document defines the roadmap for TiDB development.
 
 ## TiKV:
 
-- [ ] Raft
-    - [x] Region merge
-    - [ ] Local read thread
-    - [ ] Multi-thread raftstore
-    - [x] None voter
-    - [x] Pre-vote
-    - [ ] Multi-thread apply pool
-    - [ ] Split region in batch
-    - [ ] Raft Engine
-- [x] RocksDB 
-    - [x] DeleteRange
-    - [ ] BlobDB 
-- [x] Transaction
-    - [x] Optimize transaction conflicts
-    - [ ] Distributed GC
-- [x] Coprocessor
-    - [x] Streaming
-- [ ] Tool
-    - [x] Import distributed data
-    - [ ] Export distributed data
-    - [ ] Disaster Recovery
-- [ ] Flow control and degradation
+### Raft
+
+- [x] Region Merge - Merge small Regions together to reduce overhead
+- [x] Local Read Thread - Process read requests in a local read thread
+- [x] Split Region in Batch - Speed up Region split for large Regions
+- [x] Raft Learner - Support Raft learner to smooth the configuration change process
+- [x] Raft Pre-vote - Support Raft pre-vote to avoid unnecessary leader election on network isolation
+- [ ] Joint Consensus - Change multi members safely.
+- [ ] Multi-thread Raftstore - Process Region Raft logic in multiple threads
+- [ ] Multi-thread apply pool - Apply Region Raft committed entries in multiple threads
+
+### Engine
+
+- [ ] Titan - Separate large key-values from LSM-Tree
+- [ ] Pluggable Engine Interface - Clean up the engine wrapper code and provide more extendibility
+
+### Storage
+
+- [ ] Flow Control - Do flow control in scheduler to avoid write stall in advance
+
+### Transaction
+
+- [x] Optimize transaction conflicts
+- [ ] Distributed GC - Distribute MVCC garbage collection control to TiKV
+
+### Coprocessor
+
+- [x] Streaming - Cut large data set into small chunks to optimize memory consumption
+- [ ] Chunk Execution - Process data in chunk to improve performance
+- [ ] Request Tracing - Provide per-request execution details
+
+### Tools
+
+- [x] TiKV Importer - Speed up data importing by SST file ingestion
+
+### Client
+
+- [ ] TiKV client (Rust crate)
+- [ ] Batch gRPC Message - Reduce message overhead
 
 ## PD:
 
@@ -69,7 +86,7 @@ This document defines the roadmap for TiDB development.
 - [x] Decentralize scheduling table Regions
 - [x] Scheduler supports prioritization to be more controllable
 - [ ] Use machine learning to optimize scheduling
-- [ ] Cluster Simulator
+- [ ] Optimize Region metadata - Save Region metadata in detached storage engine
 
 ## TiSpark: