Fast gc

[heidawei]

this PR for issue #42531

1. I check the key which has too many versions to GC, than I use fast GC to process, I will delete the old versions with delete range.
   another, I will compute the MVCC stats, it will take many time, so i need hijack the context to ignore timeout(maybe we need discuss)

2. I check hot key which has too many versions to GC. to reduce latency

[cockroach-teamcity]

This change is 

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[ajwerner]

I've implemented effectively this here: #51230. I am going to close this as I don't think it's going to get merged.