Implement utility functions for loose index scan

What does this MR do and why?

This MR implements a solution to address the distinct counting (and iteration) problems on unbounded scopes (no time range filter). The method (if used correctly) will a read maximum DISTINCT_ITEMS_IN_COLUMN number of rows.

Example 1: distinct authors in the events table. How many users generated events? (usage ping)

count = 0
Event.include(EachBatch)
Event.distinct_each_batch(column: 'author_id', of: 10000) { |r| count += r.count(:author_id) }

The snippet above took 23 minutes on db-lab.

Example 2: Pulling the first 100 authors

Issue.include(LooseIndexScan)
Issue.loose_index_scan(column: :author_id).limit(100)

Example 3: Distinct project_id in Ci::Pipeline

• Plan with DISTINCT: https://explain.depesz.com/s/YiaJ
• Plan with loose index scan: https://explain.depesz.com/s/fgku

MR acceptance checklist

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