Enhancement

The codes under "dbms/src/Storages/Transaction" are some codes we adapt TiFlash with TiDB. After years of development, it become kind of crowded and confusing.

Suggest breakdown "dbms/src/Storages/Transaction” into these modules:

• Some global context for TiFlash management
• Basic structure from TiDB (TableInfo/DatabaseInfo, etc)
• Syncing schema with TiDB
• Decoding data from row to column (Block)
• Raft layer
  • Region info
  • Handling admin command
  • Handling write command
  • Handling apply snapshot/ingest sst
  • Handling learner read

1. Suggest creating a dbms/src/TiDB as the module for basic components from TiDB. And
   • Rename TMTContext to TiFlashContext
   • Move TableInfo/DBInfo, etc from TiDB.h to dbms/src/TiDB
   • Move TiFlashContext/ManagedStorages to the dbms/src/TiDB module
   • Add a dbms/src/TiDB/Codec and move the logic of decoding data from row to column there
   • Add a dbms/src/TiDB/Schema and move schema syncing logic there
2. Rename dbms/src/Transaction to dbms/src/KVStore. And maybe we should further break down the codes for different Raft processes under this module.
   • Rename dbms/src/Transaction to dbms/src/KVStore Refactor kvstore #8073
   • Break down KVStore. Refactor kvstore #8073