Support compact all data by SQL

[flowbehappy]

Feature Request

There are chances that the data in TiFlash is not very optimized. Like background GC workload failed to catch up cleaning old data after extending nodes. It causes too much outdate data in old TiFlash nodes, which affect query performance. Currently, we need to use tools like ch client to run SQLs to manually compact the data, which is very inconvenient.

Consider providing SQL which can execute directly from TiDB:

alter table T compact tiflash replica; 

[JaySon-Huang]

should take care if T is a partition table, then it will trigger compaction on all partitions

[breezewish]

Implemented in #4885