Numeric field index refactoring. Required changes for #2666

struct NumericIndex was converted into enum NumericIndex with only one variant Mutable (#2666 intriduces Immutable variant, I hope in future we will implement Mmap variant too).

Mutable variant contains all fields from the old struct NumericIndex, and has the same load/delete/add methods as the old ones. All filters/estimations are the same but with changed access to old structure fields. Now it was added new methods to access Mutable fields:

NumericIndex::get_db_wrapper
NumericIndex::get_histogram
NumericIndex::get_points_count
NumericIndex::get_values_count
NumericIndex::get_values
NumericIndex::get_max_values_per_point

Also the whole file numeric_index.rs was separated into parts with NumericIndex, mutable state, and tests. It's the same strategy that was applied to Map Index.

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