Tracking issue for making region size dynamic

[BusyJay]

RFC: tikv/rfcs#82

Summary:

Make the size of a region dynamic, and only set a upper limit.

This is the first step that we try to support PB scale cluster.

Tasks:

• Tracking issue for making region size dynamic pd#4428
• PD schedules well on both size balance and load balance.
• TiKV supports buckets #11759
• Standalone streaming for reporting buckets flow #11718
• PD uses buckets to do hotspot split and deprecated TiKV local hotspot split
• TiKV side concurrency for coprocessor requests #11976
• Warming up in pretransfer leader
• Infinite leader lease #11579
• Lightening/BR presplit reconfiguration based on the new size
• Multiple file snapshot support #11595
• (Optional) more efficient snapshot generation and apply

The feature is expected to be disabled by default when release as it may bring extra replication cost in current architecture.

The task item may change as the proposal is not approved yet.

[bufferflies]

Can these tasks be closed?