storage: push down columns to update within a column family.

[andreimatei]

Currently, updates to some columns in a column family is implemented by reading the whole column family (in fact, all the col fams until #18168 is fixed) in order to compute the value for the col fam. But we shouldn't need to read anything; kv should be able to decode the columns sufficiently so that it can only overwrite the intended columns.
We should be careful to always update the timestamp cache narrowly so that updates to a non-overlapping set of columns contend as little as possible.

[andreimatei]

Extracted morally from #20080 which talks more generally about pushing down various types of computation.

[andreimatei]

Actually I guess for blind writes the timestamp cache is not updated, so I was talking nonsense about it above.

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