skip store limit for empty regions created by lightning

[mittalrishabh]

Enhancement Task

Lightning creates thousands of empty regions. Lightning hit the store limit very often and it sometimes lead to skewed regions.
We should skip the store limit if scatter on empty regions are inexpensive.

[rleungx]

We have added an option to skip the limit check: #6593

[mittalrishabh]

I want to understand how it impacts tikv if there are lot of pending scatter operations. Do you have a write up on it

[rleungx]

If we don't scatter the regions or the scatter operation is pending/canceled, the import job may have hotspots on TiKV. Most of the import flow might happen on the three TiKVs if we only use the 3 replicas setting, which affects its progress and the data might be skewed on TiKVs.

[mittalrishabh]

Lightning with parallel import can cause split operations on non empty regions. I want to understand the implication of that.