Avoid calculating the sum of items in bitmap statistics

[Zachary Dremann]

Calculating the sum of values requires visiting every item in the bitmap: Other than this, all other statistics can be calculated by only visiting each container. Is it worth either:

1. Removing the field entirely (breaking change, both API and ABI)
2. Changing the implementation to leave sum as zero, and documenting that the sum field is not actually updated.

Originally posted by @AviAvni in #617 (comment)

[Avi Avni]

we can also provide another api to get a sum just like min and max and it can be optimize for example if we can iterate on ranges for example 1..100 1000..2000 we can just simply calculate (1+100)*100/2+(1000+2000)*1000/2

[Daniel Lemire]

I think it would be an acceptable breaking change.