Fix distincted exhaustive hits

[ManyTheFish]

This PR changes the name and behavior of bucket_candidates:

• bucket_candidates become initial_candidates that is less confusing
• initial_candidates is no more a simple RoaringBitmap but an enum allowing us to precise if the candidates are exhaustive or not
• this enum ensures that any modification is allowed only if the candidates are not already exhaustive.

The bug occurred because initial_candidates are modified during the bucket sort allowing the estimation to be more and more precise along the search, and this was an issue when the initial_candidates were already exhaustive, now, if candidates are exhaustive, then no modifications are made.

[bors[bot]]

Build succeeded:

• Run Clippy
• Run Rustfmt
• Tests on macos-latest
• Tests on ubuntu-20.04
• Tests on windows-latest