What problem does this PR solve?

Issue Number: close #4646

Problem Summary:

The Transaction folder is renamed to KVStore, since it is originally a KVStore with a percolator executor to decode row data into column data built on raft kv.

The new KVStore folder will be made into a new kvstore library which will be linked to dbms.

The following files will be moved to TiDB:

1. Collator
2. TiDB row/column format. These codes are purely definitions of TiDB encoding, including Datum and RowCodec and TypeMapping. It does not relate to TiKV key's encoding.

The KVStore folder includes:

1. Storages/KVStore/Decode is a bridge between KVStore and DeltaMerge. It is about:
   • How we read keys and values from KVStore.
   • How we use utils from TiDB and decode the value in row format kvs.
   • How we will transform row format into column format. It includes modules like RegionTable.
2. Storages/KVStore/FFI is FFI utils and interfaces with TiFlash Proxy.
3. Storages/KVStore/MultiRaft contains all modules that implements a raft kv. Including region, region manager, region range index.
4. Storages/KVStore/Utils
5. Storages/KVStore/TiKVHelpers contains some util functions about definitions in TiKV which can be extract as common util function, such as key format for percolator/keyspace, and pd client. The key format part is about how to parse TiKVKey, and uses some information in TiKVValue to perform percolator 2pc commit. Note some TiKV key utils are related to schema or range of tables, we will put them in Storages/KVStore/Decode.
6. Storages/KVStore/Read contains all about learner read with strong consistency, inclduing mvcc and region lock info and read index. It does not include read from data KVStore and ingest into DeltaMerge, which is in Storages/KVStore/Decode.

The whole classification is not accurate, since we still rely on some decoded data such as DecodedTiKVKey in MultiRaft.

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None