storage: extract post-commit portion of split
storage: extract post-commit portion of split

This is the first step towards formalizing the split trigger as a side effect
in the sense of #6286. With proposed-evaluated KV, we will have to attach
a small amount of metadata to the WriteBatch containing the SplitTrigger based
from which we have to deduce the necessary actions.

I have more WIP which contains plumbing-type work, but have decided to take
a step back and bring these changes in in small increments as a plethora of
roadblocks is to be expected and #6144 may influence (and/or be influenced by)
this work.

Code movement only (and changing a few words around the boundary of the move)
in this commit.

petermattis/cockroach,sunya123/cockroach,paperstreet/cockroach,sunya123/cockroach,paperstreet/cockroach,maxlang/cockroach,maxlang/cockroach,maxlang/cockroach,arjun4084346/cockroach,maxlang/cockroach,vaibhavarora/cockroach,glycerine/cockroach,glycerine/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach,maxlang/cockroach,glycerine/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,sunya123/cockroach,paperstreet/cockroach,cockroachdb/cockroach,cockroachdb/cockroach,arjun4084346/cockroach,cockroachdb/cockroach,veteranlu/cockroach,arjun4084346/cockroach,veteranlu/cockroach,arjun4084346/cockroach,glycerine/cockroach,veteranlu/cockroach,glycerine/cockroach,sunya123/cockroach,vaibhavarora/cockroach,arjun4084346/cockroach,cockroachdb/cockroach,veteranlu/cockroach,arjun4084346/cockroach,veteranlu/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach,vivekmenezes/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,veteranlu/cockroach,glycerine/cockroach,arjun4084346/cockroach,veteranlu/cockroach,vivekmenezes/cockroach,vivekmenezes/cockroach,vivekmenezes/cockroach,petermattis/cockroach,petermattis/cockroach,veteranlu/cockroach,petermattis/cockroach,vivekmenezes/cockroach,vaibhavarora/cockroach,cockroachdb/cockroach,glycerine/cockroach,petermattis/cockroach,vaibhavarora/cockroach,petermattis/cockroach,sunya123/cockroach,cockroachdb/cockroach,sunya123/cockroach,sunya123/cockroach,paperstreet/cockroach,petermattis/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,petermattis/cockroach,paperstreet/cockroach,glycerine/cockroach,sunya123/cockroach,vivekmenezes/cockroach,maxlang/cockroach,vaibhavarora/cockroach,maxlang/cockroach,arjun4084346/cockroach,maxlang/cockroach