Add levels to DocValues skipper index

[iverase]

Currently the DocValues skipper index collects the stats every 4096 documents that allow implementors to used them to decide if they want to process those documents or they can be skipped. The idea of adding levels is to be able to skip several of those block (called intervals in the code) in one step by collecting the stats of the blocks and adding them to the index.

This implementation collects the stats of 8 of those intervals. For the first level, the first interval gets a new level with the stats from the next 8 intervals, then the 9th interval gets another level with the stats from the following 8 intervals and so on. For the second level, the first interval gets a new level with the stats from the next 8 level1 intervals, which is the same as the stats from the first 64 level 0 intervals and so on.

I run some basic experiments and I must say I did not see much change on the performance, still it feels the right thing to do, therefore I opened this PR.

relates #11432

[jpountz]

It makes sense to me overall.

[jpountz]

Thanks, it does indeed make more sense to me to expose all levels. I left a few comments.

[jpountz]

What was wrong with having this check on every level?

[iverase]

The current condition is skipper.minDocID(level) < doc where the doc is the provided doc on advance. So the provided doc needs to be lower than or equal to the minDocID not to trigger the error. I

It checks we have advance to the closer interval forward. This check is strange as it is only valid for implementations with exact bounds and it is clearly not valid for levels above 0.

[jpountz]

This makes sense.

[jpountz]

This makes sense.