To standalone functions in index/secondary_index_manager.{hh,cc}. This
way, alternative data dictionary implementations (in
tools/schema_loader.cc), can also re-use this code without having to
instantiate a database or resorting to copy-paste.
The functions are slighly changed: there are some additional params
added to cover for things not internally available in the database
object. const sstring& is converted to std::string_view.
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