Query all unused indexes.

Data is retrieved from the performance_schema database and the table_io_waits_summary_by_index_usage table. The database should be running for a while since unused indexes are tracked based on IO activity. An index is considered unused when it has 0 hits and almost unused when it has 50 hits or less. The latter can however be configured by passing: min_hits as an argument. Also indexes of small tables (less than 5 pages) are excluded. Primary keys are excluded from the query.