Enhancement

During the huge transactions, memory usage of TiFlash rises quickly.
Currently, TiFlash caches all uncommitted transaction data in memory. For example, a transaction with over 100GB updates would like to cause OOM.

• TiDB stores row values in Lock cf Percolator. And since TiFlash does not use the values, we can safely remove them. optimize memory usage of lock cf #4857
• Flush Memtable and Persist Region operations could block the processing of apply logs, which cause more entry cache of raft logs